Soft-fail on network errors

Soft-fail on network errors

When an update could not be performed because of a network
error (host unreachable/no route to host) we should not
log an error to the database and hold off updates for a
long time.

Those errors are usually caused by local configuration
errors and the servers of the providers should not be
bothered with update requests as those won't reach them
any way.

The client will try to resend the update at the next
update call.

Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>