Avoid redirect loop if backend is not reachable

[georgeto]

The exception handler for BackendErrors redirects to the index page.
If the user is logged in, and the BackendError is caused by the backend not being reachable, the flask login user loader will again raise a BackendError and with that comes the next redirect...

Somehow we have to detect this scenario and return an anonymous user from our user loader, and at the time display a Communication with the backend server failed error message.