Fix edge cases leading to backup hanging forever

Plug a couple of apparent code paths (one not obviously reachable, but
fixed here on general principles) that could lead to a backup pass
getting confused partway through and simply never properly completing.
In this state it would leave its wakelock held forever until next
reboot.  Bug 5828859.

Those fixes are a total of two lines of code. The rest of the patch
adds a textual journal of the most recently completed (or ongoing!)
backup pass's progress, with an eye to being able to isolate any such
issues that may crop up in the future.

Change-Id: If8a5e8aba11db5a1e618d8b9c9ba3038dd5377a1