If somewhere in the forums, then I think “Troubleshooting” would be the most appropriate place, and “Wiki” would be the second best place.
I think the answer depends most on the target audience. New users are more likely to see it if it is in the User Guide or another page under “Setup” on the main website. More established user would be more likely to use the forums, so, if that is the target audience, it would make sense in the forums. Perhaps, it could go in the forums, but include a “Troubleshooting” link on the Setup page that goes to the Troubleshooting forum (which could then have the post stickied up top). That may catch both categories well.
That is a great point about search engines. I’ve made a page here listing all of the error messages, what they mean, and how to fix them, and I will link to it in the User Guide.