Well, wyUpdate already fixes your app when it updates (if any file fails to patch) by using the catch-all update. It's a bit beyond the scope of wyUpdate and wyBuild to let the user "rollback" to the last state at any time. That's more the job of your installer.
That being said, the line is rather blurry between an installer and the updater. We're definitely veering into the direction of adding the ability to make installers directly in wyBuild.
So the short answer is that this ability will be available when we add installer creation to wyBuild. But that's still a way off.