@Override
        public void cancel() {
            abort(subscriber instanceof AbortingSubscriber ? ((AbortingSubscriber<?>) subscriber).cause()
                                                           : CancelledSubscriptionException.get());
        }
{@link CancelledSubscriptionException} if cancelled. {@link Throwable} if aborted.
core/src/main/java/com/linecorp/armeria/common/stream/DefaultStreamMessageDuplicator.java