Change behavior when exception occurs in onNext

[mbonneau]

Right now if there is an exception in the onNext handler, it is possible that errors and exceptions get swallowed inside the library (or a promise from some other library).

The plan is to catch these and send them down the onError handler so that the user can see the problem.

Also, a default onError handler should be added that will log an error with the error information. This will help if users do not set the onError handler.

[davidwdan]

Here is a discussion about RxJS, which suffers from the same issue:
ReactiveX/rxjs#2813