Improve `executorLost` error message by including the slave id.

Reviewed at https://reviews.apache.org/r/50478/
diff --git a/src/main/java/org/apache/aurora/scheduler/mesos/MesosSchedulerImpl.java b/src/main/java/org/apache/aurora/scheduler/mesos/MesosSchedulerImpl.java
index 433fc90..df87d92 100644
--- a/src/main/java/org/apache/aurora/scheduler/mesos/MesosSchedulerImpl.java
+++ b/src/main/java/org/apache/aurora/scheduler/mesos/MesosSchedulerImpl.java
@@ -255,7 +255,7 @@
   public void executorLost(SchedulerDriver schedulerDriver, ExecutorID executorID, SlaveID slaveID,
       int status) {
 
-    log.warn("Lost executor " + executorID);
+    log.warn("Lost executor " + executorID + " on slave " + slaveID);
     counters.get("scheduler_lost_executors").incrementAndGet();
   }