Improve clarity of "adding req. component" error

Author: Jason Mobarak

src/rustup/errors.rs

@@ -50,7 +50,7 @@ error_chain! {
         }
         AddingRequiredComponent(t: String, c: Component) {
             description("required component cannot be added")
-            display("component {} is required for toolchain '{}' and cannot be re-added",
+            display("component {} was automatically added because it is required for toolchain '{}'",
                     c.description(), t)
         }
         ParsingSettings(e: Vec<toml::ParserError>) {

tests/cli-v2.rs

@@ -565,7 +565,7 @@ fn add_target_host() {
         let trip = TargetTriple::from_build();
         expect_ok(config, &["rustup", "default", "nightly"]);
         expect_err(config, &["rustup", "target", "add", &trip.to_string()],
-                   for_host!("component 'rust-std' for target '{0}' is required for toolchain 'nightly-{0}' and cannot be re-added"));
+                   for_host!("component 'rust-std' for target '{0}' was automatically added because it is required for toolchain 'nightly-{0}'"));
     });
 }