Return the SchemaElement for the specified class name, asserting that it exists.

def get_element_by_class_name_or_raise(self, class_name):
        """Return the SchemaElement for the specified class name, asserting that it exists."""
        if class_name not in self._elements:
            raise InvalidClassError(u'Class does not exist: {}'.format(class_name))

        return self._elements[class_name]