Error substate does not receive model hook rejection reason unless it is an Error object

If you return `Ember.RSVP.reject('whatever')` from a model hook, the error substate will not recieve the reason as the model as indicated in the [guides](https://guides.emberjs.com/v2.16.0/routing/loading-and-error-substates/#toc_code-error-code-substates):

> The "reason" for the error (i.e. the exception thrown or the promise reject value) will be passed to that error state as its model.

See https://ember-twiddle.com/566eb0e180a20a9f96a22578abf614d2?fileTreeShown=false&numColumns=2&openFiles=routes.application.js%2Ctemplates.application-error.hbs

If I instead return `Ember.RSVP.reject(new Error('whatever'))` it works as expected.

I think this is either a bug or a case where the docs need a bit more work.