@Override
        public String toString() {
            if (cause == null) {
                return "CloseEvent";
            } else {
                return "CloseEvent(" + cause + ')';
            }
        }
{@link CancelledSubscriptionException} if cancelled. {@link Throwable} if aborted.
core/src/main/java/com/linecorp/armeria/common/stream/DefaultStreamMessageDuplicator.java