2.x: reintroduce OnErrorNotImplementedException for 0-1 argument subscribe()

[akarnokd]

This PR reintroduces the OnErrorNotImplementedException wrapper from 1.x and applies it to the subscribe() methods that don't define an onError handler.

The errors are still routed to the RxJavaPlugins.onError handler but now wrapped with OnErrorNotImplementedException.

This should help with cases where the developer forgot to add the handler and distinguish such unhandled errors from other undeliverable errors due to lifecycle limitations.

[akarnokd]

Coverage: 95.51%

https://codecov.io/gh/ReactiveX/RxJava/pull/5036

[akarnokd]

(Messing with codecov settings, had to reopen this to trigger a webhook of theirs.)

[akarnokd]

I couldn't get Travis re-execute the job, likely due to the problems they are facing right now (even though they claim its only MacOSX builds).

[JakeWharton]

👍

[PaulWoitaschek]

So for the application usage it makes sense to re-throw everything of instance of OnErrorNotImplementedException and to log everything else?

My app became quite unstable with RxJava 2 because I'm silently swallowing errors and can't distinguish between my errors and errors thrown after I unsubscribed.

Really looking forward for this change if it does what I think it does!

[akarnokd]

If you are using subscribe() or subscribe(Consumer) and the sequence runs into an error, the error handler will receive an OnErrorNotImplementedException(originalError).

[PaulWoitaschek]

Now it's twice.

If the completable emits an error...

[PaulWoitaschek]

Once with the note about OnErrorNotImplemented..., once as swallowed

[akarnokd]

Great opportunity to post a PR!