We know that if we want to see the details of error message, we can change the
<customErrors mode="On" /> to Off and we can see the exact error message.
But wait - ever after changing this to Off in web.config file - it still shows this page below:
Well the solution is to edit the web.config file in this location with <customErrors mode="Off" /> i.e.
C:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\14\TEMPLATE\LAYOUTS
After changing the web.config, refresh the page and you will see the specific error message.