Returns an options validation error message if there are any
        invalid keywords. Otherwise returns None.

def validation_error_message(cls, spec, backends=None):
        """
        Returns an options validation error message if there are any
        invalid keywords. Otherwise returns None.
        """
        try:
            cls.validate_spec(spec, backends=backends)
        except OptionError as e:
            return e.format_options_error()