(draft) Improve error message to account for invisible ReCAPTCHA.

There are ongoing debates about good UX for failed invisible CAPTCHA errors but I don't like the existing answers. And the more I look at my suggested replacement in this MR, the less enthused I am about it. I was trying to avoid ReCAPTCHA as a term not everyone knows, but I don't know that bot is any better. And behavior requires l10n for non-US English, which feels unnecessary.

So consider this an open bikeshed question - what can we tell the user that feels actionable to them, in non-technical language? I'll mull this for a day or two, but maybe the answer is jumping out at you.