make diagnostic message clearer when called from cargo

[mathroc]

closes rust-lang/cargo#1554

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @sfackler (or someone else) soon.

If any changes to this PR are deemed necessary, please add them as extra commits. This ensures that the reviewer can see what has changed since they last reviewed the code. The way Github handles out-of-date commits, this should also make it reasonably obvious what issues have or haven't been addressed. Large or tricky changes may require several passes of review and changes.

Please see CONTRIBUTING.md for more information.

[alexcrichton]

Thanks for the PR! Our error messages conventionally start with lowercase letters instead of uppercase, so could this say "run rustc ..." instead of "Run rustc"? Other than that though this looks good to me!

[mathroc]

alright it's done. I didn't squashed it as asked from the @rust-highfive bot. if you like it better squashed as it such a small edit, just tell me :)

[alexcrichton]

r+ from me with a squash

[mathroc]

and it's squashed

[alexcrichton]

@bors: r+ 5f932e8 rollup

Thanks!

[bors]

⌛ Testing commit 5f932e8 with merge af52207...

[alexcrichton]

Looks like one of the bots disappeared and then buildbot got hung up, everything passed, though, on many platforms, so I'm going to merge this manually.