[SPARK-4737] Task set manager properly handles serialization errors

Dealing with [SPARK-4737], the handling of serialization errors should not be the DAGScheduler's responsibility. The task set manager now catches the error and aborts the stage.

If the TaskSetManager throws a TaskNotSerializableException, the TaskSchedulerImpl will return an empty list of task descriptions, because no tasks were started. The scheduler should abort the stage gracefully.

Note that I'm not too familiar with this part of the codebase and its place in the overall architecture of the Spark stack. If implementing it this way will have any averse side effects please voice that loudly.

Author: mcheah <mcheah@palantir.com>

Closes #3638 from mccheah/task-set-manager-properly-handle-ser-err and squashes the following commits:

1545984 [mcheah] Some more style fixes from Andrew Or.
5267929 [mcheah] Fixing style suggestions from Andrew Or.
dfa145b [mcheah] Fixing style from Josh Rosen's feedback
b2a430d [mcheah] Not returning empty seq when a task set cannot be serialized.
94844d7 [mcheah] Fixing compilation error, one brace too many
5f486f4 [mcheah] Adding license header for fake task class
bf5e706 [mcheah] Fixing indentation.
097e7a2 [mcheah] [SPARK-4737] Catching task serialization exception in TaskSetManager