Add assertion in `relatedTypes` to catch undefined relations

If you define a relation on a model with a String identifier of an undefined object (`"App.Usre"`, perchance) your application might appear functional until ember-data tries to sideload responses for that model. Eventually an exception is raised in the serializer's `rootForType` method, but its not immediately apparent why.

This seems like the kind of thing ember-data should have an assertion for – catching a mistake early as in `DS.JSONSerializer#sideload`.