def _validate_class(self, cl):
        """return error if class `cl` is not found in the ontology"""
        if cl not in self.schema_def.attributes_by_class:
            search_string = self._build_search_string(cl)
            err = self.err(
                "{0} - invalid class", self._field_name_from_uri(cl),
                search_string=search_string)
            return ValidationWarning(ValidationResult.ERROR, err['err'],
                                     err['line'], err['num'])
return error if class `cl` is not found in the ontology