[SPARK-1620] Handle uncaught exceptions in function run by Akka scheduler

If the intended behavior was that uncaught exceptions thrown in functions being run by the Akka scheduler would end up being handled by the default uncaught exception handler set in Executor, and if that behavior is, in fact, correct, then this is a way to accomplish that.  I'm not certain, though, that we shouldn't be doing something different to handle uncaught exceptions from some of these scheduled functions.

In any event, this PR covers all of the cases I comment on in [SPARK-1620](https://issues.apache.org/jira/browse/SPARK-1620).