Adds a convenience assertion and uses peek instead of poll for hasError

These were missing before and necessary to mark this observer as appropriately disposed after, though without calling dispose()

Saving dex methods

I'm not 100% I've done this right, but leveraging some existing examples in RxJava as my guide. I've copied in the relevant internal rxjava APIs (since they're internal) and also adjusted the onNext methods to return true or false indicating whether or not they terminated the underlying observer. Will seek further feedback during the PR. Main thing I'm not sure of is if it does terminate, do we need to dispose the lifecycle or does this mean that onError or onComplete have been hit in the main observer already (and doing so is a double dispose condition)

We were disposing the lifecycle a second time after, and unnecessarily disposing the main disposable twice too