private Tuple2<Integer, Reply> createMergedErrorReplyResult() {
        if (error != null) {
            return new Tuple2<>(null, error);
        }
        if (ignore != null && successReply == null) {
            return new Tuple2<>(null, ignore);
        }
        throw new IllegalStateException("createMergedErrorReplyResult called without error");
    }
Encapsulated logic for merging replies from 1-n related DocumentProtocol messages.
For internal use only. Not multithread safe.
documentapi/src/main/java/com/yahoo/documentapi/messagebus/protocol/ReplyMerger.java