Exception raised when SALClient given incorrect credentials could be clearer

[TOFarmer]

Summary

When invalid credentials are passed to SALClient.authenticate, the exception which is raised is sal.core.exception.InvalidResponse("The server did not return an authentication token"). It would be better if an AuthenticationFailed was raised.

Possible fixes

Ideally _handle_error could be used to address this, but raise an InvalidReponse if the response is both invalid and not a 401 error.