@Override
        public void onError(Throwable cause) {
            requireNonNull(cause, "cause");
            if (completed) {
                return;
            }
            completed = true;
            innerSubscriber.onError(cause);
            abort(cause);
        }
Relay items in order from a {@link StreamMessage} of {@link StreamMessage}s as a single {@link StreamMessage}
source.
core/src/main/java/com/linecorp/armeria/common/stream/ConcatPublisherStreamMessage.java