Extend the QAuthenticator support to handle failed authentications and to do NTML authentication based on the logged user

Extend the QAuthenticator support to handle failed authentications and to do NTML authentication based on the logged in user. For example, it should be possible to tell if the authentication will fail so that the user can be reprompted in an application or given the chance to cancel the request. Also, using the logged in user's details first for the authentication before prompting would be nice.