ExecutorScheduler delivers uncaught exceptions

Instead of swallowing unhandled errors, ExecutorScheduler delivers them
to the executing thread's UncaughtExceptionHandler.

This addresses the same issue as ReactiveX/RxJava#1682, but for
ExecutorScheduler which does not used ScheduledAction.
Merge pull request #1767 from loganj/uncaught_executor

ExecutorScheduler delivers uncaught exceptions