Editor compatibility and the new error format

[nikomatsakis]

@cuviper

I think you do really want atomic messages anyway, so parallel rustc invocations don't interleave their output badly. (Hopefully one's code isn't spitting errors all over the place, but it happens.)

Hmm. I hadn't thought about the interaction with cargo. @alexcrichton confirms that when cargo runs rustc in parallel it just spits stdout/stderr directly to the console.