If a PostDestroyEvent subscriber throws an exception, users are redirected to a WSOD

Problem/Motivation

I discovered this while working on #3338666: Add functional test that proves there is reasonable UX whenever a stage event subscriber has an exception. If a PostDestroyEvent subscriber throws an exception, the batch processor redirects you to the UpdateReady form. The first thing that form tries to do is claim the stage, but since the stage has already been destroyed, you get an exception saying "This operation was already canceled." That's no good.

Proposed resolution

If the subscribers of either PreDestroyEvent or PostDestroyEvent throw an exception, redirect to the finish page, which will report the error, and also notify the user that the update completed successfully (or needs to run update.php). In the case of PreDestroyEvent, if the user goes back to the start page and tries to start a new update, they won't be allowed to do it because the previous stage still exists...but at least they'll have the ability to delete it.