Comment 3 for bug 1437514

It's no big deal to exit Déjà-Dup while it runs. As far as I can tell, whatever backup volumes were already made are kept on the backup location, and the backup will resume later, or can be resumed manually. Or at least that's what my previous experience of Déjà-Dup has been.

I don't know if the OP is still around but I see no reason to implement a "Confirm Exit" prompt, unless I'm wrong about backups resuming of course.