Fixed the error message when cppcheck isn't installed to not be completely wrong.

Summary: Arcanist is a dirty rotten liar. I made it less of a dirty, rotten liar.

Test Plan: http://imgur.com/36qXcgI

Reviewers: Blessed Reviewers, epriestley

Reviewed By: Blessed Reviewers, epriestley

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9081