Provide user feedback for common SSH configuration issues

[qguv]

This PR introduces error messages to clarify some regular issues I encounter:

• Sup always tries to connect to a SSH_AUTH_SOCK even if one is not defined
• Sup silently continues if a defined SSH_AUTH_SOCK is unreachable
• Sup doesn't report any permissions issues preventing keys from being read
• Sup can't currently handle passphrase-encrypted keys but produces no warning about this

Dependencies needed to be updated to accomplish this last task, and a small change was required to maintain the existing (lack of) SSH hostname check behavior.