Return the exception class suitable for the specified HTTP
        status code.

        Raises:
            UnknownError: The HTTP status code is not one of the knowns.

def _exception_for(self, code):
        """Return the exception class suitable for the specified HTTP
        status code.

        Raises:
            UnknownError: The HTTP status code is not one of the knowns.
        """
        if code in self.errors:
            return self.errors[code]
        elif 500 <= code < 599:
            return exceptions.RemoteServerError
        else:
            return exceptions.UnknownError