This PR adds logic to our fallback exception handler to render werkzeug HTTPException instances.

Previously, calling internal resources (or custom routes that would not be detected as s3 routes) with wrong methods would output:

{
  "error": "Unexpected exception",
  "message": "405 Method Not Allowed: The method is not allowed for the requested URL.",
  "type": "MethodNotAllowed"
}

because it interprets the MethodNotAllowed exception as unexpected exception. with this new logic that generalize capturing of werkzeug HTTP exceptions, you can actually raise HTTP exceptions from within handlers, and get proper exception messages:

{
  "error": "Method Not Allowed",
  "message": "The method is not allowed for the requested URL."
}

Unfortunately, because the service name parser is still executed before the router handler, it doesn't work for most requests, since those are captured by the ServiceExceptionSerializer which will serialize exceptions into s3 exceptions although the calls are not being made to s3. I added a bunch of tests & xfailed tests to document the expected behavior once that is remedied.