Clean up some errors in tor-dirmgr

This branch cleans up some error messages and types in tor-dirmgr to be more informative.

I'm marking this as Draft because we shouldn't merge it before resolving #521 (closed) and putting out a tor-dirmgr patch release.

There are a couple of bigger refactorings that I am not doing right now:

• Including full DocSource information in every error that comes from a document.
• Describing what kind of a document we were trying to get in every applicable error.
• Separating the errors that apply only to a single bootstrap-driven operation from those that refer to an API-level call.

I think those would all be fine down the line, but they make more sense after other planned dirmgr refactoring like #433.