Add error simulation to Dummy Auth

Errors are triggered using the Error_Code url parameter (to align with
the ID_User parameter already in place).  Fixes #144.

Also, add the nested HybridAuth exception to the HttpError thrown by the
Auth class when it picks up a HybridAuth error in the authenticate
function.  NOTE: errors on login are not thrown from here (and transformed
into HttpErrors) - they emerge from /Class/Auth/Auth.class.php#L140