Marking non-existent challenges

[laz0r]

This is a shoutout to adum - would it be possible to mark the challenges accessed with:
www.hacker.org/challenge/chal.php?id=
so that if a challenge doesn't exist (as of writing, for example, id=1000 doesn't exist), the page contains some sort of error message? This is specifically in relation to dangermouse's script http://www.hacker.org/forum/viewtopic.php?t=2426
so that it could automatically find the highest challenge.
I would have thought this would be easy to do - just have a flag "active", "deleted" or default "nonexistent" attached to each puzzle.