rustdoc: integrate a proper warning/error handler

[QuietMisdreavus]

The current markdown rendering warnings are just a collection of println!() statements. It would be worthwhile to create a rustc_errors::Handler for rustdoc to properly emit errors from, so that these and others can be given proper formatting, and possibly even a mini lint system, or at least being able toggle specific errors between allow/warn/err.

[euclio]

I think this is fixed, no?

[QuietMisdreavus]

Indeed it is! We've started using a diagnostic emitter from rustc_errors, and all our errors and warnings go through that mechanism now. I'll close this.