Distinguish between errors that don't exist and those that don't yet have an entry

There's some error numbers that are generated by the compiler, but don't yet have an entry on the error message index.

When searching for these errors, nothing comes up. This makes me feel like I've copied over the message wrong.

It would be helpful if even if didn't have an entry that a placeholder one would turn up that could explain that it's a valid message but that no one has written up an entry yet and maybe suggesting to the user that it's something they could do.