Added warning message. Which, sadly, also meant bringing in lager as dependency (wanted to keep `riak_ensemble` generic and not depend on lager, but I'm too lazy to deal with things now and took the easy route). We do want to start with an empty backend state though, since we're designed to handle that case. The peer will come up untrusted, but after syncing with other peers, will have a perfectly valid state once again (assuming a majority of remaining have valid states -- which is an inherent assumption in `riak_ensemble` anyway).