Add details when BlockManager heartbeats time out

Makes it more clear what the threshold was for tuning spark.storage.blockManagerSlaveTimeoutMs

Before:
WARN  "Removing BlockManager BlockManagerId(201304022120-1976232532-5050-27464-0, myhostname, 51337) with no recent heart beats

After:
WARN  "Removing BlockManager BlockManagerId(201304022120-1976232532-5050-27464-0, myhostname, 51337) with no recent heart beats: 19216ms exceeds 15000ms

core/src/main/scala/spark/storage/BlockManagerMasterActor.scala

@@ -121,7 +121,8 @@ class BlockManagerMasterActor(val isLocal: Boolean) extends Actor with Logging {
     val toRemove = new HashSet[BlockManagerId]
     for (info <- blockManagerInfo.values) {
       if (info.lastSeenMs < minSeenTime) {
-        logWarning("Removing BlockManager " + info.blockManagerId + " with no recent heart beats")
+        logWarning("Removing BlockManager " + info.blockManagerId + " with no recent heart beats: "
+          (now - info.lastSeenMs) + "ms exceeds " + slaveTimeout + "ms")
         toRemove += info.blockManagerId
       }
     }