[SPARK-23806] Broadcast.unpersist can cause fatal exception when used…

… with dynamic allocation

## What changes were proposed in this pull request?

ignore errors when you are waiting for a broadcast.unpersist. This is handling it the same way as doing rdd.unpersist in https://issues.apache.org/jira/browse/SPARK-22618

## How was this patch tested?

Patch was tested manually against a couple jobs that exhibit this behavior, with the change the application no longer dies due to this and just prints the warning.

Please review http://spark.apache.org/contributing.html before opening a pull request.

Author: Thomas Graves <[email protected]>

Closes apache#20924 from tgravescs/SPARK-23806.

Author: Thomas Graves

core/src/main/scala/org/apache/spark/storage/BlockManagerMasterEndpoint.scala

@@ -192,11 +192,15 @@ class BlockManagerMasterEndpoint(
     val requiredBlockManagers = blockManagerInfo.values.filter { info =>
       removeFromDriver || !info.blockManagerId.isDriver
     }
-    Future.sequence(
-      requiredBlockManagers.map { bm =>
-        bm.slaveEndpoint.ask[Int](removeMsg)
-      }.toSeq
-    )
+    val futures = requiredBlockManagers.map { bm =>
+      bm.slaveEndpoint.ask[Int](removeMsg).recover {
+        case e: IOException =>
+          logWarning(s"Error trying to remove broadcast $broadcastId", e)
+          0 // zero blocks were removed
+      }
+    }.toSeq
+
+    Future.sequence(futures)
   }
 
   private def removeBlockManager(blockManagerId: BlockManagerId) {