Possible to change wordpress error page (not 404)

Hi There,
I have just implemented the Pay to Blog feature within prosites, I have a few sites that are unpaid and so, as it should, it displays the Frontend Restricted message on the default wordpress error page.

I'd really like to customize this error page entirely - change the background, fonts etc, but also, most importantly, change the page title so it doesn't say "Wordpress > Error"

Is there a way to create custom error pages (All I've found in search results is about 404 pages, but this isn't one of those)
Thanks
Sam