Remove errant in arcanist.php

Summary: This causes 49 to be printed out preceding the start of my next terminal line every time there is an exception thrown.

Test Plan: did not see 49 printed out when exceptions were thrown

Reviewers: Blessed Reviewers, epriestley

Reviewed By: Blessed Reviewers, epriestley

Subscribers: epriestley

Differential Revision: https://secure.phabricator.com/D14030