[FR] Add some state around describers

[paullewis]

When we use a board-as-component we load its descriptor on the fly. However, we can land in a funny place if the board being loaded is unavailable because the user is – say – offline. We don't really have any form of state for that, but I think if we had something we could query on the node, e.g., "described", "pending", "failed" then we could adapt the UI to it.