Fatal System.err Logs on Unhandled Exceptions

If an exception is thrown on a thread then we can't do anything with it so will log out to System.err.

Fixes https://github.com/ReactiveX/RxJava/issues/1682 ScheduledAction Swallows Errors
Merge pull request #1746 from benjchristensen/issue-1682

Fatal System.err Logs on Unhandled Exceptions