[SPARK-3747] TaskResultGetter could incorrectly abort a stage if it cannot get result for a specific task

Author: Reynold Xin <rxin@apache.org>

Closes #2599 from rxin/SPARK-3747 and squashes the following commits:

a74c04d [Reynold Xin] Added a line of comment explaining NonFatal
0e8d44c [Reynold Xin] [SPARK-3747] TaskResultGetter could incorrectly abort a stage if it cannot get result for a specific task