We used to raise exceptions in the controllers
and then handle them with exception apps.

This was best tested with blocks of the form
```
  assert_not_found do
    get :show, id: :nonexisting
  end

```

Since we handle the error cases directly now
we can test for the response code instead.

This allows a simpler style of assertions:
```
  get :show, id: :nonexisting
  assert_not_found
```

This is more chronological, less lines and easier to understand.