If a PostRequireEvent subscriber throws an exception, users are redirected to the confirmation page

Problem/Motivation

This is yet another bug discovered while working on #3338666: Add functional test that proves there is reasonable UX whenever a stage event subscriber has an exception.

It turns out that, if a PostRequireEvent subscriber throws an exception, the user is still redirect to the confirmation page to continue the update (/admin/automatic-update-ready). They will see the error message, and they won't be able to continue the update, but what's bad here is that this is very incongruous UX, because if a PreRequireEvent subscriber throws an exception, users are redirected to the start page!

Proposed resolution

If a PostRequireEvent subscriber throws an exception, the stage should be destroyed and the user should be redirected back to the start.