@Override
        public void onError(Throwable cause) {
            requireNonNull(cause, "cause");
            if (isCancelled()) {
                return;
            }

            if (error) {
                return;
            }
            error = true;

            downstream.onError(cause);
            publisher.abort(cause);
        }
Sets the {@link OuterSubscriber}.
This should be set before subscribing to upstream.
core/src/main/java/com/linecorp/armeria/common/stream/ConcatPublisherStreamMessage.java