[Countdown] fix undefined variable errors

Summary:
I was trying out the "Countdown" addon and noticed this undefined variable error.

##[Wed Feb 05 03:28:02 2014] [error] [client 97.88.250.50]   #0 PhutilErrorHandler::handleError(8, Undefined variable: errors, /phabricator/src/applications/countdown/controller/PhabricatorCountdownEditController.php, 132, Array of size 12 starting with: { request => Object AphrontRequest }) called at [/phabricator/src/applications/countdown/controller/PhabricatorCountdownEditController.php:132], referer: http://phabricator.eatstreet.com/countdown/##

Test Plan:
http://phabricator.eatstreet.com/countdown/edit/

No error

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley

CC: Korvin, epriestley, aran

Differential Revision: https://secure.phabricator.com/D8143