Show lint/unit failure explanation in Phabricator

Summary:
Tweaked lint/unit field specifications to introduce the failure
explanation read from arc:[lint|unit]-excuse.

Task ID: #

Blame Rev:

Test Plan:
Create dumb diffs with errors - run modified 'arc' and change
conduit_url to http://phabricator.dev1020.facebook.com/api/ - verified
that explanation shows up with proper formatting.

Revert Plan:

Tags:

Reviewers: epriestley, nh

Reviewed By: epriestley

CC: Girish, akramer, blair, aran, epriestley, andreygoder

Differential Revision: https://secure.phabricator.com/D1689