Improve newsletter subscription CAPTCHA error feedback

As discussed on IRC:

lavamind  when the captcha is wrong, the feedback message appears under the audio player, and is somewhat small
lavamind  there's no feedback around the captcha field itself
lavamind  like on the main donate form
lavamind  I think I went into the javascript rabbit hole when trying to fix that yesterday
lavamind  could we have the "is-invalid" class applied on the capctah input field when the page loads after failing the captcha verification?