Return the schema element with the given name, asserting that it's of edge type.

def get_edge_schema_element_or_raise(self, edge_classname):
        """Return the schema element with the given name, asserting that it's of edge type."""
        schema_element = self.get_element_by_class_name_or_raise(edge_classname)

        if not schema_element.is_edge:
            raise InvalidClassError(u'Non-edge class provided: {}'.format(edge_classname))

        return schema_element