Add help message if sun is sent an invalid command
Add help message if sun is sent an invalid command

Summary: This also makes "sun, rollback" redundant, so I removed it.

Test Plan: fingers crossed.

Reviewers: mroth, benjaminpollack

Reviewed By: benjaminpollack

Differential Revision: https://phabricator.khanacademy.org/D21780

Khan/slack-deploy-hooks,Khan/slack-deploy-hooks