Presently, if I neglect to start a cluster, go to mariadb and attempt to work with columnstore data, I get:
ERROR 1815 (HY000): Internal error: DBRM is not responding. Cannot accept queries.
We should be able to find out that the reason "DBRM is not responding" is that the cluster has not been started, and give an appropriate message.
Todd Stoffel would write the message text for you.