Fix SPARK-4471: blockManagerIdFromJson function throws exception while B...

Fix [SPARK-4471](https://issues.apache.org/jira/browse/SPARK-4471): blockManagerIdFromJson function throws exception while BlockManagerId be null in MetadataFetchFailedException

let's even make a `MetadataFetchFailException` and call `toTaskEndReason` on it to simulate the real issue