[autotest] Log exceptions raised in special tasks.

BUG=chromium:369079
TEST=Ran special task that was throwing exception, saw output.

Change-Id: Idacc6fc3927a5908da33d13222eb9121d812bd0e
Reviewed-on: https://chromium-review.googlesource.com/198046
Commit-Queue: Alex Miller <milleral@chromium.org>
Tested-by: Alex Miller <milleral@chromium.org>
Reviewed-by: Dan Shi <dshi@chromium.org>
4 files changed