Remove accidental sprintf injection in error reporting

Summary:
STDERR output with %s in it could cause:

ERROR 2: fprintf(): Too few arguments at [/usr/local/arcanist/src/workflow/ArcanistFeatureWorkflow.php:170]

Test Plan: Untested.

Reviewers: Blessed Reviewers, epriestley

Reviewed By: Blessed Reviewers, epriestley

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D19261