Throw error if improperly defined factory

Basically throws a better error then something like:

```js
Error while processing route: managedExercise.edit undefined is not a function TypeError: undefined is not a function
    at instantiate (http://localhost:3000/scripts/application.js:15865:26)
    at lookup (http://localhost:3000/scripts/application.js:15731:19)
    at Object.Container.lookup (http://localhost:3000/scripts/application.js:15400:16)
    at EmberObject.extend.controllerFor (http://localhost:3000/scripts/application.js:36957:32)
    at EmberObject.extend.setup (http://localhost:3000/scripts/application.js:36472:31
```

Which is usually an incorrect export of a class from a module, at least when working with browserify. The factory comes back as `{}` and doesn't have a `create`, so it dies.. since it's truthy.