R1:330d62984b7c

Check required classes in setup

Test Plan:
Run setup with 'differential.attach-task-class' set to:

- ''
- 'FacebookTasksAttacher'
- 'X'

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, epriestley

Differential Revision: https://secure.phabricator.com/D1999