We all know that Microsoft blew it big time with this update. To bad they wouldn't just own up to it, and admit it.
It's not just this update, they're repeating the same thing for August. After fighting with KB2919355 for months and finally getting it to install, I've now got to the August version of KB2919355, KB2975719. This grinds away for what seems like an hour and then reports (you guessed it) "We couldn’t complete the updates, Undoing changes".
It seems like Microsoft have learned absolutely nothing from the KB2919355 mess, the latest KB2975719 has exactly the same issue.