In IIS Manager, select votre site, double-cliquez le Error Pages icon et cliquez sur Edit Feature Settings in le pane on le right. Choose Detailed Errors. If that is pas enough, cliquez sur le .NET Error Pages in le ASP.NET section, again Edit Feature Settings in le pane on le right, choose Off.
If this is a production machine, changez lese settings back après you are done avec debugging votre code.