Reroute exceptions in onNext to onError by default

[mbonneau]

Fixes #192

This PR catches exceptions in onNext in the default observer that is created when you call subscribe using callbacks and sends them to the onError handler.

This resolves issues with errors and exceptions happening in a completely silent way.

This may not resolve silent failures if you do not have a handler for the error. This can be dealt with in a separate PR after that warns if you do not pass in an onError and an error occurs.

This is specifically only applied to the default observer that is built in subscribe. If the user sends in their own Observer the library leaves handling these issues up to the user.

[coveralls]

Coverage increased (+0.003%) to 99.4% when pulling 537deec on mbonneau:reroute-exceptions into a5d7c1e on ReactiveX:master.