Make error types usable in more contexts

[joshlf]

Per #1288 (comment):

Error types like SizeError do not implement std::error::Error, so the functions like read_from_bytes() that now return a Result can't be used with anyhow::Context, while the previous version that returned Option could. Since Debug and Display are already implemented, I think this should just be a matter of adding impl std::error::Error for SizeError {}, but I didn't actually try this out. It may still not be possible to call .context() due to other constraints (Send + Sync). This is easy enough to work around (assuming the caller doesn't really care about the underlying error), and we didn't use this pattern in many places, so it's not really a big deal for us.

Implementing std::error::Error is easy, and is prototyped in #1298, pending changes described in #1298 (comment).

Supporting Send + Sync may be harder, since the contained Src type may not be Send or Sync. We may need to provide a way to discard the Src type, and we will need to consider how discoverable this mechanism is for people who are encountering a trait-not-satisfied error. I could see a few options:

• Provide an Error<Src> -> Error<()> conversion
• Provide a new type alias type Foo = Error<()> so that type signatures are cleaner
• Provide an entirely new type which contains similar information to the original error, but without the source
• Provide an entirely new type which contains the name of the source type, but doesn't contain an instance of the source type (just like we do for Dst right now)