Introduce forgiving version of DefaultExceptionHandler (w.r.t. channel closure)

[michaelklishin]

DefaultExceptionHandler treats unhandled consumer exceptions as fatal, that is, closes the channel. I'm not quite sure this is what most people would expect from an exception handler.

Perhaps we should not close the channel, only println the exception.

@dumbbell @videlalvaro what do you think?

[michaelklishin]

The plan is to introduce a new more forgiving ExceptionHandler and base the default one on it in 3.5.x, then change the default for 3.6.0.

[michaelklishin]

Fixed by #74.