Improve the quality of SSH error messages

Summary: See PHI1784. Currently, users who pass an invalid SSH command to Phabricator's SSH handler get an unhelpful error message. Make it more helpful.

Test Plan: Ran ./bin/ssh-exec with no arguments (old, helpful error), invalid arguments (before: unhelpful error; after: helpful error), and valid arguments (old, helpful behavior).

Differential Revision: https://secure.phabricator.com/D21362