[SPARK-11195][CORE] Use correct classloader for TaskResultGetter

Make sure we are using the context classloader when deserializing failed TaskResults instead of the Spark classloader.

The issue is that `enqueueFailedTask` was using the incorrect classloader which results in `ClassNotFoundException`.

Adds a test in TaskResultGetterSuite that compiles a custom exception, throws it on the executor, and asserts that Spark handles the TaskResult deserialization instead of returning `UnknownReason`.

See #9367 for previous comments
See SPARK-11195 for a full repro

Author: Hurshal Patel <hpatel516@gmail.com>

Closes #9779 from choochootrain/spark-11195-master.