I'd be very happy to change this - I'm not sure that the current logic is correct.

The [current logic](https://github.com/twpayne/ol3/blob/026192d6118c390e161058127d4b22e2a1f8c304/src/ol/layer/layergroup.js#L221-L238) is that the layer group takes on the *worst* state of its children:
- if *any* member of a layer group is in the `ERROR` state then the entire layer group is in the `ERROR` state
- otherwise, if *any* member of the layer group is in the `LOADING` state then the entire layer group is in the `LOADING` state
- otherwise, the layer group is in the `READY` state

It might be better to give the layer group the *best* state of any of its children.