Remove "error" qualifier from error messages, as it's redundant

compatibility-tests/compile-fail/tests/ui/attribute-misuse.stderr

@@ -1,58 +1,58 @@
-error: `display` attribute is only valid on variants of an error enum, not on an enum
+error: `display` attribute is only valid on variants of an enum, not on an enum
  --> $DIR/attribute-misuse.rs:5:13
   |
 5 |     #[snafu(display("display should not work here"))]
   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
-error: `source(from)` attribute is only valid on fields of an error variant, not on an enum
+error: `source(from)` attribute is only valid on fields of a variant, not on an enum
  --> $DIR/attribute-misuse.rs:6:13
   |
 6 |     #[snafu(source(from(XXXX, Box::new)))]
   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
-error: `source(bool)` attribute is only valid on fields of an error variant, not on an enum
+error: `source(bool)` attribute is only valid on fields of a variant, not on an enum
  --> $DIR/attribute-misuse.rs:7:13
   |
 7 |     #[snafu(source(true))]
   |             ^^^^^^^^^^^^
 
-error: `backtrace` attribute is only valid on fields of an error variant, not on an enum
+error: `backtrace` attribute is only valid on fields of a variant, not on an enum
  --> $DIR/attribute-misuse.rs:8:13
   |
 8 |     #[snafu(backtrace)]
   |             ^^^^^^^^^
 
-error: `context` attribute is only valid on an error variant, not on an enum
+error: `context` attribute is only valid on a variant, not on an enum
  --> $DIR/attribute-misuse.rs:9:13
   |
 9 |     #[snafu(context)]
   |             ^^^^^^^
 
-error: `source` attribute is only valid on fields of an error variant, not on a variant
+error: `source` attribute is only valid on fields of a variant, not on a variant
   --> $DIR/attribute-misuse.rs:21:46
    |
 21 |         #[snafu(display("an error variant"), source(from(XXXX, Box::new)))]
    |                                              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
-error: `source` attribute is only valid on fields of an error variant, not on a variant
+error: `source` attribute is only valid on fields of a variant, not on a variant
   --> $DIR/attribute-misuse.rs:22:17
    |
 22 |         #[snafu(source)]
    |                 ^^^^^^
 
-error: `backtrace` attribute is only valid on fields of an error variant, not on a variant
+error: `backtrace` attribute is only valid on fields of a variant, not on a variant
   --> $DIR/attribute-misuse.rs:23:17
    |
 23 |         #[snafu(backtrace)]
    |                 ^^^^^^^^^
 
-error: `display` attribute is only valid on variants of an error enum, not on a field
+error: `display` attribute is only valid on variants of an enum, not on a field
   --> $DIR/attribute-misuse.rs:34:21
    |
 34 |             #[snafu(display("display should not work here"))]
    |                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
-error: `visibility` attribute is only valid on an error enum and its variants, not on a field
+error: `visibility` attribute is only valid on an enum and its variants, not on a field
   --> $DIR/attribute-misuse.rs:35:21
    |
 35 |             #[snafu(visibility(pub))]
@@ -64,37 +64,37 @@ error: Incompatible attributes [`source(false)`, `source(from)`] specified on a
 37 |             #[snafu(source(from(XXXX, Box::new)))]
    |                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
-error: `context` attribute is only valid on variants of an error enum, not on a field
+error: `context` attribute is only valid on variants of an enum, not on a field
   --> $DIR/attribute-misuse.rs:38:21
    |
 38 |             #[snafu(context)]
    |                     ^^^^^^^
 
-error: `display` attribute is only valid on variants of an error enum, not on a struct
+error: `display` attribute is only valid on variants of an enum, not on a struct
   --> $DIR/attribute-misuse.rs:51:13
    |
 51 |     #[snafu(display("display should not work here"))]
    |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
-error: `visibility` attribute is only valid on an error enum and its variants, not on a struct
+error: `visibility` attribute is only valid on an enum and its variants, not on a struct
   --> $DIR/attribute-misuse.rs:53:13
    |
 53 |     #[snafu(visibility(pub))]
    |             ^^^^^^^^^^^^^^^
 
-error: `source(bool)` attribute is only valid on fields of an error variant, not on a struct
+error: `source(bool)` attribute is only valid on fields of a variant, not on a struct
   --> $DIR/attribute-misuse.rs:54:13
    |
 54 |     #[snafu(source(true))]
    |             ^^^^^^^^^^^^
 
-error: `backtrace` attribute is only valid on fields of an error variant, not on a struct
+error: `backtrace` attribute is only valid on fields of a variant, not on a struct
   --> $DIR/attribute-misuse.rs:55:13
    |
 55 |     #[snafu(backtrace)]
    |             ^^^^^^^^^
 
-error: `context` attribute is only valid on variants of an error enum, not on a struct
+error: `context` attribute is only valid on variants of an enum, not on a struct
   --> $DIR/attribute-misuse.rs:56:13
    |
 56 |     #[snafu(context)]

compatibility-tests/compile-fail/tests/ui/duplication-backtrace-delegate.stderr

@@ -1,11 +1,11 @@
-error: Cannot have `backtrace` field and `backtrace` attribute on a source field in the same error variant
+error: Cannot have `backtrace` field and `backtrace` attribute on a source field in the same variant
   --> $DIR/duplication-backtrace-delegate.rs:10:9
    |
 10 | /         #[snafu(backtrace)]
 11 | |         source: String,
    | |______________________^
 
-error: Cannot have `backtrace` field and `backtrace` attribute on a source field in the same error variant
+error: Cannot have `backtrace` field and `backtrace` attribute on a source field in the same variant
  --> $DIR/duplication-backtrace-delegate.rs:7:9
   |
 7 |         backtrace: Backtrace,

compatibility-tests/compile-fail/tests/ui/duplication-backtrace-fields.stderr

@@ -1,4 +1,4 @@
-error: Multiple `backtrace` attributes are not supported within an error variant
+error: Multiple `backtrace` attributes are not supported within a variant
   --> $DIR/duplication-backtrace-fields.rs:10:17
    |
 10 |         #[snafu(backtrace)]

compatibility-tests/compile-fail/tests/ui/duplication-enum.stderr

@@ -1,4 +1,4 @@
-error: Multiple `visibility` attributes are not supported on an error enum
+error: Multiple `visibility` attributes are not supported on an enum
  --> $DIR/duplication-enum.rs:5:9
   |
 5 | #[snafu(visibility(pub))]

compatibility-tests/compile-fail/tests/ui/duplication-multiple-attributes.stderr

@@ -1,4 +1,4 @@
-error: Multiple `visibility` attributes are not supported on an error variant
+error: Multiple `visibility` attributes are not supported on a variant
  --> $DIR/duplication-multiple-attributes.rs:6:30
   |
 6 |     #[snafu(visibility(pub), visibility(pub))]

compatibility-tests/compile-fail/tests/ui/duplication-source-attributes.stderr

@@ -1,4 +1,4 @@
-error: Multiple `source` attributes are not supported within an error variant
+error: Multiple `source` attributes are not supported within a variant
   --> $DIR/duplication-source-attributes.rs:10:18
    |
 10 |          #[snafu(source)]

compatibility-tests/compile-fail/tests/ui/duplication-source-from.stderr

@@ -1,4 +1,4 @@
-error: Multiple `source` attributes are not supported within an error variant
+error: Multiple `source` attributes are not supported within a variant
   --> $DIR/duplication-source-from.rs:10:18
    |
 10 |          #[snafu(source(from(EnumError, Box::new)))]

compatibility-tests/compile-fail/tests/ui/duplication-struct.stderr

@@ -1,4 +1,4 @@
-error: Multiple `source(from)` attributes are not supported on an error struct
+error: Multiple `source(from)` attributes are not supported on a struct
  --> $DIR/duplication-struct.rs:8:9
   |
 8 | #[snafu(source(from(UsableError, Box::new)))]

compatibility-tests/compile-fail/tests/ui/duplication-variant.stderr

@@ -1,4 +1,4 @@
-error: Multiple `display` attributes are not supported on an error variant
+error: Multiple `display` attributes are not supported on a variant
  --> $DIR/duplication-variant.rs:6:13
   |
 6 |     #[snafu(display("should not allow duplicate display"))]

snafu-derive/src/lib.rs

@@ -381,7 +381,7 @@ fn parse_snafu_enum(
 
     let mut errors = SyntaxErrors::default();
 
-    let mut default_visibilities = AtMostOne::new("visibility", "on an error enum");
+    let mut default_visibilities = AtMostOne::new("visibility", "on an enum");
 
     for attr in attributes_from_syn(attrs)? {
         match attr {
@@ -393,7 +393,7 @@ fn parse_snafu_enum(
                     tokens,
                     OnlyValidOn {
                         attribute: "display",
-                        valid_on: "variants of an error enum",
+                        valid_on: "variants of an enum",
                         not_on: "an enum",
                     },
                 );
@@ -406,7 +406,7 @@ fn parse_snafu_enum(
                                 tokens.clone(),
                                 OnlyValidOn {
                                     attribute: "source(bool)",
-                                    valid_on: "fields of an error variant",
+                                    valid_on: "fields of a variant",
                                     not_on: "an enum",
                                 },
                             );
@@ -416,7 +416,7 @@ fn parse_snafu_enum(
                                 tokens.clone(),
                                 OnlyValidOn {
                                     attribute: "source(from)",
-                                    valid_on: "fields of an error variant",
+                                    valid_on: "fields of a variant",
                                     not_on: "an enum",
                                 },
                             );
@@ -429,7 +429,7 @@ fn parse_snafu_enum(
                     tokens,
                     OnlyValidOn {
                         attribute: "backtrace",
-                        valid_on: "fields of an error variant",
+                        valid_on: "fields of a variant",
                         not_on: "an enum",
                     },
                 );
@@ -440,7 +440,7 @@ fn parse_snafu_enum(
                     OnlyValidOn {
                         // TODO: enums for these
                         attribute: "context",
-                        valid_on: "an error variant",
+                        valid_on: "a variant",
                         not_on: "an enum",
                     },
                 );
@@ -460,9 +460,9 @@ fn parse_snafu_enum(
             let name = variant.ident;
             let variant_span = name.span();
 
-            let mut display_formats = AtMostOne::new("display", "on an error variant");
-            let mut visibilities = AtMostOne::new("visibility", "on an error variant");
-            let mut contexts = AtMostOne::new("context", "on an error variant");
+            let mut display_formats = AtMostOne::new("display", "on a variant");
+            let mut visibilities = AtMostOne::new("visibility", "on a variant");
+            let mut contexts = AtMostOne::new("context", "on a variant");
             let mut doc_comment = String::new();
             let mut reached_end_of_doc_comment = false;
 
@@ -476,7 +476,7 @@ fn parse_snafu_enum(
                             tokens,
                             OnlyValidOn {
                                 attribute: "source",
-                                valid_on: "fields of an error variant",
+                                valid_on: "fields of a variant",
                                 not_on: "a variant",
                             },
                         );
@@ -486,7 +486,7 @@ fn parse_snafu_enum(
                             tokens,
                             OnlyValidOn {
                                 attribute: "backtrace",
-                                valid_on: "fields of an error variant",
+                                valid_on: "fields of a variant",
                                 not_on: "a variant",
                             },
                         );
@@ -522,8 +522,8 @@ fn parse_snafu_enum(
             };
 
             let mut user_fields = Vec::new();
-            let mut source_fields = AtMostOne::new("source", "within an error variant");
-            let mut backtrace_fields = AtMostOne::new("backtrace", "within an error variant");
+            let mut source_fields = AtMostOne::new("source", "within a variant");
+            let mut backtrace_fields = AtMostOne::new("backtrace", "within a variant");
 
             for syn_field in fields {
                 let original = syn_field.clone();
@@ -627,7 +627,7 @@ fn parse_snafu_enum(
                                 tokens,
                                 OnlyValidOn {
                                     attribute: "visibility",
-                                    valid_on: "an error enum and its variants",
+                                    valid_on: "an enum and its variants",
                                     not_on: "a field",
                                 },
                             );
@@ -637,7 +637,7 @@ fn parse_snafu_enum(
                                 tokens,
                                 OnlyValidOn {
                                     attribute: "display",
-                                    valid_on: "variants of an error enum",
+                                    valid_on: "variants of an enum",
                                     not_on: "a field",
                                 },
                             );
@@ -647,7 +647,7 @@ fn parse_snafu_enum(
                                 tokens,
                                 OnlyValidOn {
                                     attribute: "context",
-                                    valid_on: "variants of an error enum",
+                                    valid_on: "variants of an enum",
                                     not_on: "a field",
                                 },
                             );
@@ -713,11 +713,11 @@ fn parse_snafu_enum(
                     let backtrace_location = backtrace.1.clone();
                     errors.add(
                         source_location,
-                        "Cannot have `backtrace` field and `backtrace` attribute on a source field in the same error variant",
+                        "Cannot have `backtrace` field and `backtrace` attribute on a source field in the same variant",
                     );
                     errors.add(
                         backtrace_location,
-                        "Cannot have `backtrace` field and `backtrace` attribute on a source field in the same error variant",
+                        "Cannot have `backtrace` field and `backtrace` attribute on a source field in the same variant",
                     );
                 }
                 _ => {} // no conflict
@@ -789,7 +789,7 @@ fn parse_snafu_struct(
 ) -> MultiSynResult<StructInfo> {
     use syn::Fields;
 
-    let mut transformations = AtMostOne::new("source(from)", "on an error struct");
+    let mut transformations = AtMostOne::new("source(from)", "on a struct");
 
     let mut errors = SyntaxErrors::default();
 
@@ -800,7 +800,7 @@ fn parse_snafu_struct(
                     tokens,
                     OnlyValidOn {
                         attribute: "display",
-                        valid_on: "variants of an error enum",
+                        valid_on: "variants of an enum",
                         not_on: "a struct",
                     },
                 );
@@ -810,7 +810,7 @@ fn parse_snafu_struct(
                     tokens,
                     OnlyValidOn {
                         attribute: "visibility",
-                        valid_on: "an error enum and its variants",
+                        valid_on: "an enum and its variants",
                         not_on: "a struct",
                     },
                 );
@@ -823,7 +823,7 @@ fn parse_snafu_struct(
                                 tokens.clone(),
                                 OnlyValidOn {
                                     attribute: "source(bool)",
-                                    valid_on: "fields of an error variant",
+                                    valid_on: "fields of a variant",
                                     not_on: "a struct",
                                 },
                             );
@@ -837,7 +837,7 @@ fn parse_snafu_struct(
                     tokens,
                     OnlyValidOn {
                         attribute: "backtrace",
-                        valid_on: "fields of an error variant",
+                        valid_on: "fields of a variant",
                         not_on: "a struct",
                     },
                 );
@@ -847,7 +847,7 @@ fn parse_snafu_struct(
                     tokens,
                     OnlyValidOn {
                         attribute: "context",
-                        valid_on: "variants of an error enum",
+                        valid_on: "variants of an enum",
                         not_on: "a struct",
                     },
                 );
@@ -1388,7 +1388,7 @@ impl<'a> quote::ToTokens for ContextSelector<'a> {
 
                 let names: Vec<_> = user_fields.iter().map(|f| f.name.clone()).collect();
                 let selector_doc = format!(
-                    "SNAFU context selector for the `{}::{}` error variant",
+                    "SNAFU context selector for the `{}::{}` variant",
                     enum_name, variant_name,
                 );