I'm getting ready to do an upgrade for my site, and I'm going to be taking it down while I upload files and upgrade the database.

While this is happening, a runtime error will happen on the site. I've adjusted the web.config to
<customErrors defaultRedirect="~/maintenance.htm" mode="On">
There is no detailed information being displayed, but my custom page is not being displayed either. I'm still getting a general Runtime Error and "Server Error in '/' Application."

I'm trying to see if there is something that I need to adjust in the IIS (using 7), but the google searching I've been doing haven't been very helpful. Anyone have any ideas?