Pretty tricky, the title says it all

cscript %systemdrive%\inetpub\adminiscripts\adsutil.vbs set w3svc/AspScriptErrorSentToBrowser true      (to allow error message send to client)

On a x64 OS (I run Windows 7 RC x64)

cscript C:\Windows\winsxs\amd64_microsoft-windows-iis-legacyscripts_31bf3856ad364e35_6.1.7100.0_none_4b5800ce84aa413c\adsutil.vbs set w3svc/AspScriptErrorSentToBrowser true