Refactor per-crate errors to better match plan in doc/Errors.md

We need to do #322 (closed) in %Arti 0.1.0 release: Okay for experimental embedding in order to present a stable-ish API. But after that, we'll have a lot of work left to do in order to make our per-crate errors more sensible and conformant to our design in doc/Errors.md. This will involve a lot of refactoring: but fortunately, none that will break stable APIs in arti-client.

This can be done on a crate-by-crate basis; we should cover at least everything in tor-proto and higher-level crates. Estimating around 6h per crate:

• tor-proto
• tor-netdoc
• tor-netdir
• tor-persist
• tor-chanmgr
• tor-guardmgr
• tor-circmgr
• tor-dirclient
• tor-dirmgr
• arti-client
• arti-config