void updateConfig(JRTClientConfigRequest jrtReq) {
        if ( ! responseQueue.offer(jrtReq))
            setException(new ConfigurationRuntimeException("Failed offering returned request to queue of subscription " + this));
    }
This method should ideally throw new MissingConfig/Configuration exceptions and let the caller
catch them. However, this would make the code in JRT/File/RawSource uglier.
Alternatively, it could return a SetConfigStatus object with an int and an error message.

@param jrtRequest a config request
@return an instance of a config class (subclass of ConfigInstance)
config/src/main/java/com/yahoo/config/subscription/impl/JRTConfigSubscription.java