There are two requirements for this , most articles do not mention the second requirement.
1. In the ASP configuration set " Send error to browser" to true
You can also do it using the the command-line tool AppCmd.exe
appcmd.exe set config "Default Web Site" -section:system.webServer/asp /scriptErrorSentToBrowser:"True"
http://www.iis.net/learn/application-frameworks/running-classic-asp-applications-on-iis-7-and-iis-8/classic-asp-script-error-messages-no-longer-shown-in-web-browser-by-default
2. In the Error pages Configuration go to " Edit Feature Settings" on the right & select " Detailed Errors"
http://stackoverflow.com/questions/2640526/detailed-500-error-message-asp-iis-7-5
Note this applies to all errors.
a3d18495-a1c0-459d-890b-9d72143f4781|2|5.0