Fix deadlock when full data backup times out

The code was attempting to let a reported error in the app <-> engine
surface take precedence over apparent success at the engine <-> transport
handoff surface.  However, in the case of timeout, this is inappropriate.
It was leading to deadlock because the engine runs free, with socket-closed
as its shutdown signals for determinism.  In this case that means that
having accidentally asked it to finish and report the final result, we
locked up forever since the data it was writing dutifully to the engine
was no longer being consumed, and the actual teardown signals were never
sent.

The fix is to properly express the error-state hierarchy: only when the
engine <-> transport layer is not issuing its own abort is the app-data-
moving layer consulted about errors detected at that surface.

Bug 22348852

Change-Id: I8987be0c4f708116dfeb08098d7222241ed317f3