stub: ignore unary response msg if status is not OK

[ran-su]

fixes: #5968

[ejona86]

I only just noticed, because it is a bit subtle, but we now need to handle exceptions from onNext().

Previously if onNext() threw an exception, onMessage() would propagate the exception which would cancel the call and so onClose() would be called, which would call onError().

And the code will need a bit of an ugly dance. Maybe something like:

boolean noException = false;
try {
  observer.onNext(unaryMessage);
  noException = true;
} catch (Throwable t) {
  observer.onError(t);
}
if (noException) {
  observer.onCompleted();
}