Since you've already cleared your internal storage, I assume you backed up your important data already. In that case, flash a factory image and get a fresh start.
Just don't do CM updates via the built-in updater, it's designed to work well only with CM Recovery (which is pathetically weak).
EDIT: From your feedback it seems CM Recovery is what's required to finish the update (or at least get you out of the process). Just flash it and boot into it. Replace it with a recovery of choice later.