Password reset crash with "multiple authentication backends" message

[Maximiliano Bertacchini]

SSO has logged a few instances of a crash during password reset POST, with message:

"You have multiple authentication backends configured and therefore must provide the `backend` argument or set the `backend` attribute on the user."

Haven't been able to reproduce, but looking closely at the code, it'll happen if *all* authentication backends (LaunchpadBackend, django's ModelBackend, OpenIDBackend) return `None` (verified locally by forcing a couple of `return None`). I suspect a rare combination of account+email states might be to blame (e.g. invalid, suspended, etc).

Backtrace:
ValueError: You have multiple authentication backends configured and therefore must provide the `backend` argument or set the `backend` attribute on the user.
  File "django/core/handlers/exception.py", line 41, in inner
    response = get_response(request)
  File "django/core/handlers/base.py", line 249, in _legacy_get_response
    response = self._get_response(request)
  File "django/core/handlers/base.py", line 187, in _get_response
    response = self.process_exception_by_middleware(e, request)
  File "django/core/handlers/base.py", line 185, in _get_response
    response = wrapped_callback(request, *callback_args, **callback_kwargs)
  File "django/utils/decorators.py", line 185, in inner
    return func(*args, **kwargs)
  File "webui/decorators.py", line 211, in wrapper
    return self.view_wrapper(request, fn, *args, **kwargs)
  File "webui/decorators.py", line 228, in view_wrapper
    return fn(request, *args, **kwargs)
  File "webui/views/ui.py", line 709, in reset_password
    auth.login(request, user)
  File "django/contrib/auth/__init__.py", line 150, in login
    'You have multiple authentication backends configured and '