Make exception reporting from arc be in red

Summary:
Many other status updates (such as "Builds passed!") show up
in bright background colors, making them more salient than a final
fatal "Exception".

Make exception reporting be just as colorful, so it stands out.

Test Plan:
Added an explicit throw new Exception("!!!") and saw it
in red.

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: epriestley

Differential Revision: https://secure.phabricator.com/D17748