[SPARK-21219][CORE] Task retry occurs on same executor due to race condition with blacklisting

## What changes were proposed in this pull request?

There's a race condition in the current TaskSetManager where a failed task is added for retry (addPendingTask), and can asynchronously be assigned to an executor *prior* to the blacklist state (updateBlacklistForFailedTask), the result is the task might re-execute on the same executor.  This is particularly problematic if the executor is shutting down since the retry task immediately becomes a lost task (ExecutorLostFailure).  Another side effect is that the actual failure reason gets obscured by the retry task which never actually executed.  There are sample logs showing the issue in the https://issues.apache.org/jira/browse/SPARK-21219

The fix is to change the ordering of the addPendingTask and updatingBlackListForFailedTask calls in TaskSetManager.handleFailedTask

## How was this patch tested?

Implemented a unit test that verifies the task is black listed before it is added to the pending task.  Ran the unit test without the fix and it fails.  Ran the unit test with the fix and it passes.

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

Author: Eric Vandenberg <ericvandenberg@fb.com>

Closes #18427 from ericvandenbergfb/blacklistFix.