Catch exceptions on rustc build errors

Hey Corey, one more change if you don't mind.

When rustc fails to build a crate, cargo-lite spits out a traceback. So this catches the exception and prints out a nicer error message. Also, some tools (emacs, VS and Eclipse I think, and I'm sure vim can do it, too) are able to parse the gcc, clang, rustc, etc. error reports and do things like jumping to the offending line.

So I made the output compatible with that (the `error: ` prefix before stdout was breaking it).