Handle MailboxErrors when terminating SupervisionGroups
Handle MailboxErrors when terminating SupervisionGroups

It's possible an actor we'd like to terminate has already crashed. If
so, we're done!

olleolleolle/celluloid,jstoja/celluloid,dilumn/celluloid,jasonm23/celluloid,kenichi/celluloid,marshall-lee/celluloid,davydovanton/celluloid,sideci-sample/sideci-sample-celluloid,mehta/celluloid,TiagoCardoso1983/celluloid,celluloid/celluloid,seuros/celluloid