protected void throwIfExceptionSet(ConfigSubscription<? extends ConfigInstance> sub) {
        RuntimeException subThrowable = sub.getException();
        if (subThrowable != null) {
            sub.setException(null);
            throw subThrowable;
        }
    }
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