Added a code example for emailing on 4xx and 5xx errors without 404's

Hi guys!

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | no
| Applies to    | 2.3+
| Fixed tickets | --

I thought this is useful, because this is how I actually set this up (I *do* want 403 errors emailed, for example, but definitely not all the 404 garbage).

Thanks!