private void markSubsChangedSeen(Long requiredGen) {
        for (ConfigHandle<? extends ConfigInstance> h : subscriptionHandles) {
            ConfigSubscription<? extends ConfigInstance> sub = h.subscription();
            h.setChanged(sub.isConfigChangedAndReset(requiredGen));
        }
    }
If a {@link ConfigSubscription} has its exception set, reset that field and throw it

@param sub {@link ConfigSubscription}
config/src/main/java/com/yahoo/config/subscription/ConfigSubscriber.java