fix(cli): Be more specific in what is needed

Author: epage

src/cargo/util/command_prelude.rs

@@ -452,7 +452,7 @@ pub trait ArgMatchesExt {
                 "ustc --print target-list"
             };
             bail!(
-                "\"--target\" takes an argument.
+                "\"--target\" takes a target architecture as an argument.
 
 Run `{cmd}` to see possible targets."
             );

tests/testsuite/list_availables.rs

@@ -166,7 +166,7 @@ No tests available.
             .cargo(&format!("{} --target", command))
             .with_stderr(
                 "\
-error: \"--target\" takes an argument.
+error: \"--target\" takes a target architecture as an argument.
 
 Run `[..]` to see possible targets.
 ",