Fix #18998 - Removing a backend configured in AUTHENTICATION_BACKENDS ma...

If a backend is configured in ``AUTHENTICATION_BACKENDS`` but the module is not longer available django raises a ``ImproperlyConfigured`` instead of returning an ``AnonymousUser``.

As @claudep said in first instance (https://code.djangoproject.com/ticket/18998?cnum_edit=10#comment:1), django should catch  ``ImproperlyConfigured`` and return an ``AnonymousUser`` instead.

As mentioned in (https://code.djangoproject.com/ticket/18998?cnum_edit=10#comment:2) there is another bug related to this piece of code. I've just raise another ticket (https://code.djangoproject.com/ticket/20438#ticket) in order to fix it too.