Automatic Updates: Add a rollback functionality upon installation failure, the rollback package will be available for partial-updates for automatic updates and be similar to our existing partial builds (but in reverse).
A further iteration of this is to also detect whitescreens (fatals) after a auto update, and trigger the rollback for that too.
See #22704


git-svn-id: https://develop.svn.wordpress.org/trunk@25649 602fd350-edb4-49c9-b593-d223f7449a82