Further differentiation for __DETAILSLOADED__

[stefandesu]

There is a custom property for concepts and schemes called __DETAILSLOADED__ which is only used in Cocoda. It indicates whether details for that particular entity was already loaded from the API. Previously, there were only two states (true or false). For #278, I changed it to use three states: 0 (not loaded), 1 (loaded), -1 (failed to load). Currently the -1 state is indicated by a bolt icon which shows a tooltip that no data is available for said concept.

I think we should also differentiate between "failed to load due to a network error" (which could then be retried at a later point) and "failed to load because of a non-existent URI". I'm not sure if there is currently a way to do this easily because the provider deals with network errors and the loadConcepts method in mixins/objects.js will not be able to catch that error and therefore can't know the difference between the two cases.

It is something to keep in mind for the future, not too relevant at the moment.