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"
2. In the Error pages Configuration go to " Edit Feature Settings" on the right & select " Detailed Errors"
Note this applies to all errors.