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.