Report tests results whenever possible

Summary: This adjusts the behavior of PytestTestEngine so that it correctly reports test failures natively, rather than throwing a command error.

Test Plan: Yeah right

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D10866
Patch: don't complain about reviewers not being specified.
This will break in the future when upstream arc incorporates T4631. Hopefully there will be a better solution by then
if you didn't specify a reviewer, suggest projects
if the upstream doesn't have the base commit, push it to a special ref
added --force to `arc upgrade`
send git timestamps to server