Missing a "backend" when resolving the `LOGIN_ERROR_URL` setting in the exception middleware

[offbyone]

In social_django/middleware.py, the redirect URI is resolved like this:

    def get_redirect_uri(self, request, exception):
        strategy = getattr(request, "social_strategy", None)
        return strategy.setting("LOGIN_ERROR_URL")

This doesn't pass the backend into strategy.setting() which means it doesn't load a backend-specific view.