Thank you oh wise and wonderful Frederick for posting this idea, I am sure it will brighten the lives of the users on the GMC to a point previously thought impossible.....
.....On a more serious note though, this is a really, really bad idea, for reasons already pointed out. It would soon become a cluttered forum with people posting asking completely unrelated questions, griping about GMs price and using plenty of
No need to get cheeky master caniac.

The idea isn't 'bad' per say, I just don't think it'll work... or perhaps, work that well.
I like reading error messages, and finding the solution. I've read a lot of topics in the novice Q&A, and I'm quite familiar with the various error messages which come up, and the various reasons why they would come up. It helped me grow a lot as a coder, and whenever I got a similar message in my own game making, most of the time, I would remember reading about it, and recall the solution/what caused it. Making games with GM just got a lot easier.
If there was such an error board, I'd probably read it, although I'm familiar with most of the more common error messages, now, and the different causes for them. And I think it'd be beneficial for many people new to GML coding, to read through it, too.
On a side note, Flexaplex's excellent
Novice FAQ Covers many of the most frequent, and basic error messages, and I really think that should cover it. The problem is, I don't think that many people read it.

[I know I didn't know it existed, when I started! Would have saved me hours of grief...]