Validate that a edge classname corresponds to a non-abstract edge class.

def validate_is_non_abstract_edge_type(self, edge_classname):
        """Validate that a edge classname corresponds to a non-abstract edge class."""
        element = self.get_edge_schema_element_or_raise(edge_classname)

        if element.abstract:
            raise InvalidClassError(u'Expected a non-abstract vertex class, but {} is abstract'
                                    .format(edge_classname))