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.