How to catch HTTP (4xx/5xx) errors in the global.asax file.

I'm running .net 4.5 w/ c# on IIS 8 and Win2012.
I cannot seem to get the Application_Error method to catch these errors, for example the 404 or 403. 
If I manually invoke a 404 by doing:
throw new HttpException(404, "Page Not Found");
Then the Application_Error in global.asax will catch it. However, if you navigate to an invalid url or file that doesn't exist, the global.asax will never be hit.
I've tried setting numerous things in my web.config including <customErrors ...> and <httpErrors.....> and none of them help. Any advise?
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