Here is an experience report fix for the record -
Even after being removed and re-installed Mariadb still refused to start.
/var/lib/mysql/mysql-error.log noted this problem
2018-03-21 5:51:01 140370956769472 [ERROR] InnoDB: Upgrade after a crash is not supported. This redo log was created before MariaDB 10.2.2, and we did not find a valid checkpoint. Please follow the instructions at https://mariadb.com/kb/en/library/upgrading/
2018-03-21 5:51:01 140370956769472 [ERROR] InnoDB: Plugin initialization aborted with error Generic error
2018-03-21 5:51:02 140370956769472 [Note] InnoDB: Starting shutdown…
2018-03-21 5:51:02 140370956769472 [ERROR] Plugin ‘InnoDB’ init function returned error.
2018-03-21 5:51:02 140370956769472 [ERROR] Plugin ‘InnoDB’ registration as a STORAGE ENGINE failed.
That is, to remove these /var/lib/mysql/ib_logfile* that prevented the upgrade to startup.