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
Update lint engine documentation

Summary: Fixes T6627. This class documentation is out of date and somewhat misleading in the modern environment.

Test Plan: Read it.

Reviewers: btrahan, chad

Reviewed By: chad

Subscribers: epriestley

Maniphest Tasks: T6627

Differential Revision: https://secure.phabricator.com/D10899