Return Ok status when trying to add required component

Fixes rust-lang#441

src/rustup/toolchain.rs

@@ -636,12 +636,14 @@ impl<'a> Toolchain<'a> {
                 .expect("installed manifest should have a known target");
 
             if targ_pkg.components.contains(&component) {
-                return Err(
+                // Treat it as a warning, see https://github.com/rust-lang/rustup.rs/issues/441
+                return Ok(println!(
+                    "{}",
                     ErrorKind::AddingRequiredComponent(
                         self.name.to_string(),
                         component.description(&manifest),
-                    ).into(),
-                );
+                    ),
+                ));
             }
 
             if !targ_pkg.extensions.contains(&component) {

tests/cli-v2.rs

@@ -636,7 +636,7 @@ fn add_target_host() {
     setup(&|config| {
         let trip = TargetTriple::from_build();
         expect_ok(config, &["rustup", "default", "nightly"]);
-        expect_err(config, &["rustup", "target", "add", &trip.to_string()],
+        expect_stdout_ok(config, &["rustup", "target", "add", &trip.to_string()],
                    for_host!("component 'rust-std' for target '{0}' was automatically added because it is required for toolchain 'nightly-{0}'"));
     });
 }