Custom behavior on failure

[zoten]

Hi, and thanks for the great work! One little issue: as by now, it is impossible to distinguish between a failed authentication for when RADIUS answer is negative or a failed authentication for e.g. RADIUS server offline. This means that, if the RADIUS password has changed, the system will go forever in fallback on the model authentication, and this may not be the case. Is it possible to make this behavior customizable? (subclassing is a little overkill, since you need to overwrite more than authenticate and get_server/user, since you should go until _perform_radius_auth )