Added config for error display

I was just about to deploy to my testing server when I realised that my client would see an exception on one of the test pages instead of a 404. So I added a config for showing errors,

I'm not 100% sure if this is necessary, but it was the best way I could thing of allowing my controller to drop back to displaying a 404 page if the WordPress page couldn't be found.

(P.S. there's extra lines noted in the diff due to sublime's trailing spaces plugin)