How to throw error to browser (client) for classic ASP?

Because of security concerns, IIS 7.0 has since turned off ASP's default behavior of sending script errors (including line number and code snippet to the browser. A generic error is now shown on browser for classic ASP, something like this:

An error occurred on the server when processing the URL. Please contact the system administrator.

To revert back to earlier IIS behaviour, do this on command prompt (Administrator rights may be required):

%windir%\system32\inetsrv\appcmd set config -section:asp -scriptErrorSentToBrowser:true

or

cscript adsutil.vbs set w3svc/AspScriptErrorSentToBrowser true

Hope this is helpful.