### What changes were proposed in this pull request?
When ExternalBlockStoreClient was closed, retries from RetryingBlockFetcher will cause NPE. This proposes to skip retries by RetryingBlockFetcher when ExternalBlockStoreClient is closed.
### Why are the changes needed?
When ExternalBlockStoreClient was closed, retries from RetryingBlockFetcher will cause NPE:
```
2019-10-14 20:06:16 ERROR RetryingBlockFetcher:143 - Exception while beginning fetch of 2 outstanding blocks (after 3 retries)
java.lang.NullPointerException
at org.apache.spark.network.shuffle.ExternalShuffleClient.lambda$fetchBlocks$0(ExternalShuffleClient.java:100)
at org.apache.spark.network.shuffle.RetryingBlockFetcher.fetchAllOutstanding(RetryingBlockFetcher.java:141)
at org.apache.spark.network.shuffle.RetryingBlockFetcher.lambda$initiateRetry$0(RetryingBlockFetcher.java:169)
at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511)
at java.util.concurrent.FutureTask.run(FutureTask.java:266)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
at io.netty.util.concurrent.DefaultThreadFactory$DefaultRunnableDecorator.run(DefaultThreadFactory.java:138)
```
It was happened after BlockManager and ExternalBlockStoreClient was closed due to previous errors. In this cases, RetryingBlockFetcher does not need to retry. This NPE is harmless for job execution, but is a source of misleading when looking at log. Especially for end-users.
### Does this PR introduce any user-facing change?
No
### How was this patch tested?
Existing tests.
Closes#26115 from viirya/SPARK-29469.
Lead-authored-by: Liang-Chi Hsieh <liangchi@uber.com>
Co-authored-by: Liang-Chi Hsieh <viirya@gmail.com>
Signed-off-by: Wenchen Fan <wenchen@databricks.com>
93e71e60e6