IRC Macro Handler - check result from conduit call before operating on it

Summary: 'cuz github issue 114 came into existence. instead, just return false early here. note i am not sure if I should phlog that this is happening or not but its not exception worthy IMO.

Test Plan: lint-only 'cuz i don't want to setup an IRC server locally / somehow get my local phabricator instance accessible out there. happy to test end to end if there's an easier way...!

Reviewers: vrana, epriestley

Reviewed By: vrana

CC: aran, Koolvin

Differential Revision: https://secure.phabricator.com/D2490