bug: convert DbErrors to TokenserverErrors

[ethowitz]

Description

The Tokenserver handler should always return a TokenserverError to ensure that errors are of the format expected by Tokenserver clients. In order to accomplish this, I needed to implement From<DbError> for TokenserverError. I treat any unhandled DbError as an internal error, since SQLAlchemy errors on the Python Tokenserver raise BackendErrors, which return 503s to the client.

Testing

I've added an integration test to ensure that a DbError response has the correct format

Closes #1316

[jrconlin]

I like this approach. I wonder how much I can steal for the autopush redo.

[ethowitz]

Just updated this comment to reflect the fact that we're returning 503 and not 500 for unhandled database errors

[pjenvey]

Alternatively you could take description: &str for these to avoid to_owned() at the call sites. You'll need to pass &foo when you have owned Strings and it'll add an extra clone, but that's not the common case and IMO the small clone's worth it for convenience (but feel free to disagree and leave as is).

[pjenvey]

(and when you need to avoid that extra clone you can take an Into<Cow<&str>> but it's rarely worth that effort)

[ethowitz]

I think I had done it this way to make the clones obvious: in my mind, passing by reference suggests that the function doesn't need ownership, and it may suggest to the caller that the function is less expensive than it really is. But now I'm not so sure 🤔

[ethowitz]

Perhaps having the function take Into<String> is a good compromise? It allows flexibility, but it signals to the caller that the function does in fact need ownership

[pjenvey]

I'm ok with how it is now 😄 but good point, Into<String> is simpler and works similarly and isn't too much trouble to specify as a generic type.