write_error doesn't allow for encoding of anything other than APIError subclasses

[Kevin McDermott]

        def write_error(failure):
            log.err(failure)
            if failure.check(APIError):
                status = failure.value.status
                bytes = failure.value.response
                if bytes is None:
                    bytes = self.dump_error(failure.value, request)
            else:
                bytes = str(failure.value)
                status = 500

So, there's no scope for encoding anything other than APIError subclasses, this means that, for example, psycopg2.errors pass through unencoded, and are returned directly to callers in plain text.