fix wrong error raised when private key is missing or invalid

[shashfrankenstien]

In the Connection class, if the private key file is not in key_types, or if the file path provided doesn't exist, it raises

UnboundLocalError: local variable 'key' referenced before assignment

This is because key.from_private_key_file is called in a finally block that will execute regardless of whether the try succeeds or fails. The correct branch to use is else.

Additionally, self._transport.auth_publickey and self._transport.auth_password were being called even when password and/or private_key inputs were not strings. This PR fixes this and raises CredentialException

Added required tests!