Replace ErrorTask with DocumentTask in build logger

Fixes #40

I also extended the Rustc wrapper to parse `note:` messages as warnings instead of errors.

I don't know where the code in Microsoft.VisualStudio.Project comes from, but I had to modify it and add a new reference to the `Microsoft.VisualStudio.Package.LanguageService` assembly, because tho `DocumentTask` class is defined there.
Okay, I'm going to change that. I guess `note:`s always follow another error or warning?