only dump actor backtrace if available
only dump actor backtrace if available

On JRuby actor.thread is nil for some reason?

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