Throw error when invalid SSH key is provided

[danielbachhuber]

Users can make stupid errors like accidentally deleting their SSH private key. When they do, generate a new SSH key, and forget to update Pantheon, it would be nice if Terminus threw an error or a warning in this situation:

You might be able to solve by disabling password auth over SSH too.

[joshkoenig]

+1 good idea

[stevector]

The exact error has changed a bit. We no longer fall back to a password prompt. I agree it'd be good to improve this error message.