By default, Laravel will throw a 404 error if the model binding fails. So what you'll want to do is provide a closure as a third parameter to override the behavior and throw a different error:
Route::model('pizza', 'Pizza', function()
{
throw new ModelNotFoundException; // or NotFoundHttpException or PizzaNotFoundException...
});
See: http://laravel.com/docs/routing#route-model-binding
Once you have that in place, you can catch the exception and handle it however you'd like in app/start/global.php
See: http://laravel.com/docs/errors#handling-errors
Or instead of throwing an exception, you could just use the closure to return a Redirect
response to a specific page with a flash message indicating the error.