If you're using the Load Balancing available as part of windows server then I'm afraid you won't be able to have that kind of behaviour.
As far as the load balancer is concerned the SharePoint error page is a perfectly valid HTML response regardless that it's an error and therefore it won't consider the site down.
If you want a load balancer that's application aware, then you're going to have to look at some form of hardware load balancing that's capable of testing pages/servers according to business rules. And these don't come cheap.
Paul.
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