Control Panel -> System and maintenance -> Administrative tools -> Internet Information Services Manager. Either click the Computer name for all sites, or open it up and click a particular site, then open ‘Error Pages’, right click 500 and choose Remove. Repeat for any other error pages you don't want.
This command should also do it server-wide:
%windir%\system32\inetsrv\appcmd.exe set config -section:system.webServer/httpErrors -existingResponse:PassThrough
If you're sure you want nothing to do with error pages on any of your sites, you can also completely remove the feature from Control Panel -> Programs -> Programs and features -> Turn Windows features on or off -> IIS -> WWW Services -> Common Http Features -> HTTP Errors.