[SPARK-9266] Prevent "managed memory leak detected" exception from masking original exception

When a task fails with an exception and also fails to properly clean up its managed memory, the `spark.unsafe.exceptionOnMemoryLeak` memory leak detection mechanism's exceptions will mask the original exception that caused the task to fail. We should throw the memory leak exception only if no other exception occurred.

Author: Josh Rosen <joshrosen@databricks.com>

Closes #7603 from JoshRosen/SPARK-9266 and squashes the following commits:

c268cb5 [Josh Rosen] Merge remote-tracking branch 'origin/master' into SPARK-9266
c1f0167 [Josh Rosen] Fix the error masking problem
448eae8 [Josh Rosen] Add regression test