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