Add error handling to set language command

If you type a nonexistant language an appropriate message will show,
and the language won't be changed.

helix-term/src/commands/typed.rs

@@ -1253,7 +1253,16 @@ fn language(
     }
 
     let doc = doc_mut!(cx.editor);
-    doc.set_language_by_language_id(&args[0], cx.editor.syn_loader.clone());
+
+    let loader = cx.editor.syn_loader.clone();
+    if args[0] == "text" {
+        doc.set_language(None, Some(loader))
+    } else {
+        let ok = doc.set_language_by_language_id(&args[0], loader);
+        if !ok {
+            anyhow::bail!("invalid language: {}", args[0]);
+        }
+    }
     doc.detect_indent_and_line_ending();
 
     let id = doc.id();

helix-view/src/document.rs

@@ -683,9 +683,13 @@ impl Document {
         &mut self,
         language_id: &str,
         config_loader: Arc<syntax::Loader>,
-    ) {
+    ) -> bool {
         let language_config = config_loader.language_config_for_language_id(language_id);
+        if language_config.is_none() {
+            return false;
+        }
         self.set_language(language_config, Some(config_loader));
+        true
     }
 
     /// Set the LSP.