Improve messaging around passworded SSH keys

Summary: It would be better to test if a key is passworded, but I couldn't figure out how to do that.

Test Plan: Ran "test_connection.php"

Reviewers: btrahan

Reviewed By: btrahan

CC: aran, epriestley

Maniphest Tasks: T924

Differential Revision: https://secure.phabricator.com/D1856