The error-handling in this repo grew pretty organically without us
giving much thought to an over-arching strategy. The end result of that
was a proliferation of different error types that weren't really adding
much value.
The API here is simple and the 4xx responses are really limited to bad
requests and bounce/complaint limit violations. Everything else is an
internal server error. As such, the `error` module could be simplified
by employing a blanket `Internal` error type to cover the multitude of
niche errors that should never occur during normal conditions. The
detail for those errors is still available in the `message` property and
Sentry will still log the backtrace of course.
I also noticed that the Rocket catcher stuff wasn't really being used,
so pulled that out too. We have a standard JSON error format like the
rest of the FxA ecosystem, and we always want the response payload to be
the serialisation of that structure.
One side-effect of all these changes is that the errno value has changed
in most cases. However, I took care to preserve the errno for the bounce
and complaint violations, because it's hard-coded in the auth server. It
wouldn't have been that onerous to open a PR for commensurate changes
over there, but the number of errors left here worked out in such a way
that made sense not to bother.