Readonly but with a custom error.

        The rule's arguments are validated against this schema:
        {'type': 'boolean'}

def _validate_disallowed(self, disallowed, field, value):
        """ Readonly but with a custom error.

        The rule's arguments are validated against this schema:
        {'type': 'boolean'}
        """
        if disallowed:
            msg = 'disallowed user provided config option'
            self._error(field, msg)