Presubmit spins up lots of multiprocessing processes to run
each individual test. If you cancel your presubmit run with
<ctrl>+c, that signal gets passed through to each of those,
which then raises its own KeyboardInterrupt, and prints its
own stacktrace.

This change has each member of the multiprocessing pool instead
exit gracefully (albeit with an error code) so that only the
parent process prints its stacktrace.

R=michaelpg@chromium.org

Bug: 635196
Change-Id: If9081910a359889a43bc1b72c91a859ebe37a1d6
Reviewed-on: https://chromium-review.googlesource.com/651764Reviewed-by: Robbie Iannucci <iannucci@chromium.org>
Commit-Queue: Aaron Gable <agable@chromium.org>