Testrunner doesn't catch exceptions except TestRunnererror, KeyboardInterrupt.

Unforeseen exceptions should fail the CI step.

This CL returns an error exit code for uncaught exceptions.

R=machenbach@chromium.org
CC=yangguo@chromium.org,sergiyb@chromium.org

Bug: v8:8731
Change-Id: I7fb20dad4a3eea29f1dfa87ef91d45381ee08692
Reviewed-on: https://chromium-review.googlesource.com/c/1434034
Commit-Queue: Tamer Tas <tmrts@chromium.org>
Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59278}