You’re right – we don’t document as many error codes in 2K5 as we did in 2K. Some don’t apply; others aren’t encountered as often. In fact, we didn’t document all of the 2K errors either, we just had more of them because there was so much time between releases from 2K to 2K5.
We do add new error conditions in each web release, normally for the errors we see happening most often. We normally do over 4 new web releases of Books Online a year. Make sure you are downloading the very latest Books Online and you’ll see new ones there. I have personally done 17 new error messages for the last release alone.
And you can continue to use the 2K BOL error codes, sort of. I wouldn’t trust them absolutely. If there is a newer error code in 2K5, use that, but otherwise you can use the 2K versions to point you in the right direction until we document it.