Define and implement base loading/error state handling strategy/rules

[bear-in-a-box]

As Loadable helper exists, it can somewhat define an outer shape for data that can be loaded from somewhere, be it HTTP request, file system or anything async. However, it can lead to a situation where subscriber is called more times than expected. We have to decide whether it is a good way and things should stay like this, or such states should be implemented directly in the Store class.