[SPARK-8079] [SQL] Makes InsertIntoHadoopFsRelation job/task abortion more robust

As described in SPARK-8079, when writing a DataFrame to a `HadoopFsRelation`, if `HadoopFsRelation.prepareForWriteJob` throws exception, an unexpected NPE will be thrown during job abortion. (This issue doesn't bring much damage since the job is failing anyway.)

This PR makes the job/task abortion logic in `InsertIntoHadoopFsRelation` more robust to avoid such confusing exceptions.

Author: Cheng Lian <lian@databricks.com>

Closes #6612 from liancheng/spark-8079 and squashes the following commits:

87cd81e [Cheng Lian] Addresses @rxin's comment
1864c75 [Cheng Lian] Addresses review comments
9e6dbb3 [Cheng Lian] Makes InsertIntoHadoopFsRelation job/task abortion more robust