Stale connection used after resume

Turn the backend into offline immediately when the network in unavailable,
because the 5 second (intentional) delay can be enough to re-establish
the connection to the Internet, thus keeping the backend online, even when
it should be disconnected when the network was unavailable.

Reported downstream for EWS backends at:
https://bugzilla.redhat.com/show_bug.cgi?id=1567572