[SPARK-14958][CORE] Failed task not handled when there's error deserializing failure reason

## What changes were proposed in this pull request?

TaskResultGetter tries to deserialize the TaskEndReason before handling the failed task. If an error is thrown during deserialization, the failed task won't be handled, which leaves the job hanging.
The PR proposes to handle the failed task in a finally block.
## How was this patch tested?

In my case I hit a NoClassDefFoundError and the job hangs. Manually verified the patch can fix it.

Author: Rui Li <rui.li@intel.com>
Author: Rui Li <lirui@apache.org>
Author: Rui Li <shlr@cn.ibm.com>

Closes #12775 from lirui-intel/SPARK-14958.