error

The last failure, retained until another one replaces it or the player is opened again.

Set when status becomes PlaybackStatus.Failed, and deliberately not cleared when the status moves on. An event stream replays nothing to a consumer that subscribes late, so if the snapshot forgot the error too, a fatal failure would have no record at all for anyone who was not already listening. State conflates and is therefore the right place for the one fact a consumer must not miss.