Improve error messages on unexpected API responses

[carlobeltrame]

Cases to cover:

• A relation is accessed, but the API yields a primitive value chore: error message improvement #259
• A relation is accessed, but the API yields no value for this relation name chore: error message improvement #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.