Don't exit when slave lost because it may not be our tasks' slave.

Exit was added because it was thought it caused the program to hang and
consume resources in mesos otherwise. This is not the case as later
commits show that printLogs() was actually causing the process to block
and then ResourceOffers() was consuming for really an unknown reason :/.

This change has been tested to be safe (not consume all mesos resources)
when:

* Random slaves are lost in cluster.
* Slave the task is running on is lost.
* Slave the task is running on becomes "unhealthy" (simulated with
  iptables).

main.go

@@ -71,7 +71,7 @@ func (sched *MesosRunonceScheduler) FrameworkMessage(_ sched.SchedulerDriver, ei
 	log.Errorf("framework message from executor %q slave %q: %q", eid, sid, msg)
 }
 func (sched *MesosRunonceScheduler) SlaveLost(_ sched.SchedulerDriver, sid *mesos.SlaveID) {
-	log.Exitf("slave lost: %v", sid)
+	log.V(1).Infof("slave lost: %v", sid)
 }
 func (sched *MesosRunonceScheduler) ExecutorLost(_ sched.SchedulerDriver, eid *mesos.ExecutorID, sid *mesos.SlaveID, code int) {
 	log.Errorf("executor %q lost on slave %q code %d", eid, sid, code)