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.
Sometimes this is useful, though, because the `note` might have another span, e.g. for borrowcheck errors, there are notes that point to the conflicting regions (try this [example](https://gist.github.com/Boddlnagg/5b2ad19722bdced23fa6)). When they are reported as errors, VS adds squiggle tags and lets one double click on that "note". I also tried to use `LogMessage` instead of `LogWarning` but these messages didn't appear anywhere then.