Distinguish D-Bus timeouts when configuring printing via debugd.

Previously, if we failed to communicate with debugd, we gave ourselves a
single opaque error value. To better gain insight into these communication
errors, we add extra handling to the D-Bus callbacks to record specific
timeouts or non-responses.

R=skau@chromium.org, stevenjb@chromium.org, thestig@chromium.org

Bug: 929772
Change-Id: Ic56fff645c2fbc078559b4dc37b33f589415ef87
Reviewed-on: https://chromium-review.googlesource.com/c/1473823
Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Reviewed-by: Sean Kau <skau@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: Kalvin Lee <kdlee@chromium.org>
Cr-Commit-Position: refs/heads/master@{#634016}
8 files changed