@Override
        public void onError(Throwable throwable) {
            requireNonNull(throwable, "throwable");
            abortUnsubscribedSources(throwable);
            downstream.onError(throwable);
        }
Relay items in order from subsequent {@link StreamMessage}s as a single {@link StreamMessage} source.
core/src/main/java/com/linecorp/armeria/common/stream/ConcatArrayStreamMessage.java