void invokeOnSubscribe0() {
            try {
                subscriber.onSubscribe(this);
            } catch (Throwable t) {
                unsubscribe(t);
                throwIfFatal(t);
                logger.warn("Subscriber.onSubscribe() should not raise an exception. subscriber: {}",
                            subscriber, t);
            }
        }
{@link CancelledSubscriptionException} if cancelled. {@link Throwable} if aborted.
core/src/main/java/com/linecorp/armeria/common/stream/DefaultStreamMessageDuplicator.java