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
Merge branch 'master' of github.com:phacility/arcanist

* 'master' of github.com:phacility/arcanist:
  Report tests results whenever possible