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
Correct defect in Dummy auth

Was setting the user ID during the loginStart, but in real providers
it's not done until the end (once the external call returns and provides
an identifier that can be correlated with a user account).  The issue
was causing meant a login failure in DummyAuth was still switching the
PHP.Gt userID even though HybridAuth never accepted that the user was
authenticated.
Merge branch '144-dummy-oauth-error-simulation' of git://github.com/8w/PHP.Gt