RxWorkflowNode causes CancellationException

[CherryPerry]

1. Setup proper exception handling (either real app or RxError JUnit rule)
2. Subscribe to waitForChildAttached when child is not and and won't be attached
3. Do not unsubscribe
4. Invoke onDestroy of the node
5. See CancellationException

Single should not use takeUntil for disposing subscription, it triggers CancellationException because Single is forced to send either item or error.

[CherryPerry]

@zsoltk Actually, what behaviour do we want? If we silently dispose subscription when one of the nodes in the workflow becomes unavailable then it might be unexpected, especially when you are treating workflow as Completable which leads to onComplete callback when it was canceled. If we throw error, then it should be handled by a client (at least swallowed with empty onError block).

Would prefer error with better description.

[zsoltk]

We can offer two similarly named methods for the two use cases. One for "If this child attaches, I'll want to do something with it; if it doesn't, I don't care", one for "I expect this child to be attached; if it doesn't, it's an issue".