improve error message for npm-based formatters to emphasize local-only

Hello dear maintainers,

I was setting up spotless in my project and while trying it out I started facing the below exception (which I already managed to fix).

I got a bit scared though, because it seems that it's trying to send my code to a remote server... is that correct?

I understand that you don't have control over all the plugins (i.e. I guess nothing would prevent prettier itself from doing this), but in this case here it seems that potential code leaking is coming from spotless itself.

If the answer to the above question is "yes", is it possible to optout of this altogether?

The below exception seems to be related to [this class](https://github.com/diffplug/spotless/blob/main/lib/src/main/java/com/diffplug/spotless/npm/SimpleRestClient.java), but I can't make any conclusions just by looking at this class alone.

Thanks!