Automatic updates: An error code containing 'do_rollback' can be used to trigger a rollback.

This could enable the triggering of a rollback in update_core() in wp-admin/includes/update-core.php.

see #22704.


git-svn-id: https://develop.svn.wordpress.org/trunk@25782 602fd350-edb4-49c9-b593-d223f7449a82