private void replyError(Request request, Version version, Protocol protocol, int traceLevel, Error err) {
        Reply reply = new EmptyReply();
        reply.setContext(new ReplyContext(request, version, protocol));
        reply.getTrace().setLevel(traceLevel);
        reply.addError(err);
        handleReply(reply);
    }
Send an error reply for a given request.

@param request    The JRT request to reply to.
@param version    The version to serialize for.
@param traceLevel The trace level to set in the reply.
@param protocol   The message protocol to serialize with.
@param err        The error to reply with.
messagebus/src/main/java/com/yahoo/messagebus/network/rpc/RPCSend.java