Fixed 404 handling in app generator.

Return 404 instead of 500 for show action on unknown resource id.

Signed-off-by: Magne Land <magne.land@appfolio.com>
there's no need to define the not_found template, as it is given by the framework. 
But! for such a simple app, the autoloader is being too lazy so by the time the resource definition is read the template is not yet registered (i.e. this is a bug).

I've pushed a change that solve the issue. So if you don't mind pulling the latest code and resubmitting the PR without this template block?