Based on my reading of the threads, the error message is beyond the average technical knowhow of many club level admins. A plain English error message would therefore appear to be a neat solution.
I have tweaked the error message a little bit, but any presumption that somehow this very technical issue can be explained using strictly non-technical terms is problematic and unrealistic. Support for the use of custom domains (and free HTTPS certs for those domains) is an *optional*, advanced feature of FreeToastHost, and does require someone in the clubs using that feature to have a bit more technical know-how. We do the best we can to make things as "plain English" as possible, but there are some features that require a bit more technical understanding. This is why we put appropriate warnings in our doc about custom domains.