Don't cancel the entire backup if agent disconnects and log if the agent pipe is broken

Normally the bug fix and the extra logging should be in separate CLs.
However as explained in the linked bug, there is a race condition
between the method throwing the IOException returning and us getting a
cancellation callback from ActivityManager. If we get the callback
first, the session fails. The extra IPC from the logging in handling the
exception means the callback gets a few additional milliseconds to win
the race. So with the logging, the race condition is much more likely to
happen and the logging should not be checked in without the bug fix. And
if one gets rolled back, the other should as well. This is why I made
them a single CL.

Flag: EXEMPT bugfix
Bug: 400669414
Test: manual // PFTBT is notoriously impossible to write unit tests for.
Change-Id: Icbcb72a876f927a94675c9837b0206f1a631dff5