Provide N1QL utility method to deal with finalSucess -> rows or errors flow

When dealing with asynchronous N1QL results, it is often needed to branch from the finalSuccess and then do something with either rows or errors.

Sometimes it would be less tedious to have a utility method abstract that away, and to let errors surface as an Exception in the onError of the Observable (eg. expose rows if finalSuccess is true but raise an exception if finalSuccess is false).

It could be interesting to provide quality-of-life utility methods that do that by flatMapping on the finalSuccess and then either process rows or emit onError with the content of the errors() observable.