update_do_one($module, $number, $dependency_map, &$context)
Implements callback_batch_operation().
Performs one update and stores the results for display on the results page.
If an update function completes successfully, it should return a message as a string indicating success, for example:
return t('New index added successfully.');
Alternatively, it may return nothing. In that case, no message will be displayed at all.
If it fails for whatever reason, it should throw an instance of Drup