Limitations from having 'forward_max_recipients_error' as a config setting

I'm testing the 8.x-2.2 release. I'm now kinda wishing #2855645: Re-allow multiple recipients didn't go into the 8.x-2.x branch. :/

It seems fairly strange that forward_max_recipients_error is a config setting. Some limitations I see from this:

1. It really wants to use TranslationInterface::formatPlural(), which is impossible as a config setting.
2. It's only translatable if you go through all the trouble to have separate translations of your forward.settings config entity, which is non-trivial.
3. Needlessly bloats the settings UI.

I don't see any discussion at #2855645 about why this is the way it is. I vote for ripping this setting out and having a hard-coded (but easily translatable) formatPlural() that doesn't need "recipient(s)" and the like.

While we're at it, instead of $form_state->setErrorByName('', $message); why not set the validation error on the form element that's invalid? I.e. $form_state->setErrorByName('recipient', $message); (or whatever it should be). ;)

Thoughts?

Thanks!
-Derek