Improve error messages on unexpected API responses

Cases to cover:
* [x] A relation is accessed, but the API yields a primitive value #259
* [x] A relation is accessed, but the API yields no value for this relation name #259
* [ ] A primitive value is accessed, but the API yields a relation
* [ ] A primitive value is accessed, but the API yields no value for this property name

These cases usually result in obscure error messages like `t is not a function`, but only after the data has loaded from the API, i.e. inside a Promise.then(), which is really hard to understand for the hal-json-vuex user.

Start by tracing the Promise.then() path where API data is loaded and inspect how relation and primitive values are accessed. Cover the two unchecked cases: a primitive value receiving a relation, and a primitive value missing its property name; done means both produce understandable errors instead of messages such as `t is not a function`.

Written by the indexing model from the issue text.