run_tests: improve exit code behavior

Rather than throw an exception when pytest itself exits non-zero,
pass that back up.  The traceback is never useful, only confusing.

Change-Id: I0cd7bea730e13c9969154326057196295e550843
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/247175
Reviewed-by: David Pursehouse <dpursehouse@collab.net>
Tested-by: Mike Frysinger <vapier@google.com>