Get the location name from a location that is expected to point to a vertex.

def _get_vertex_location_name(location):
    """Get the location name from a location that is expected to point to a vertex."""
    mark_name, field_name = location.get_location_name()
    if field_name is not None:
        raise AssertionError(u'Location unexpectedly pointed to a field: {}'.format(location))

    return mark_name