WordPress - If we hit a "permission denied" error, return HTTP 403

WordPress - If we hit a "permission denied" error, return HTTP 403

Overview
--------

On WordPress, any CiviCRM page that produces a "permission denied" sends an
HTTP status code of 200.  This is incorrectly indicates that the page-request was a success.

Before
------

```
$ curl -vv 'http://wpmaster.127.0.0.1.nip.io:8001/civicrm/dashboard' 2>&1 |grep HTTP
> GET /civicrm/dashboard HTTP/1.1
< HTTP/1.1 200 OK
```

After
-----

```
$ curl -vv 'http://wpmaster.127.0.0.1.nip.io:8001/civicrm/dashboard' 2>&1 |grep HTTP
> GET /civicrm/dashboard HTTP/1.1
< HTTP/1.1 403 Forbidden
```