@Override
        public void onError(Throwable t) {
            subscriber.onError(t);
            if (currentExposedBodyPartPublisher != null) {
                currentExposedBodyPartPublisher.abort(t);
            }
            cleanup();
        }
For hiding Subscriber interface from MultipartDecoder.
Subscribing {@code DecodedHttpStreamMessage<BodyPart>} and delegating to external subscriber.
core/src/main/java/com/linecorp/armeria/common/multipart/MultipartDecoder.java