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