R1:bee69f9ce209

do not re-use output when an exception occurs

Summary:
$stdout from the previous run would be reused if an exception
occurred

Test Plan: that's a negative, ghostrider.

Reviewers: epriestley, jungejason

Reviewed By: epriestley

CC: aran, epriestley

Differential Revision: https://secure.phabricator.com/D2008