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.

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git-svn-id: http://core.svn.wordpress.org/trunk@25695 1a063a9b-81f0-0310-95a4-ce76da25c4cd