MDL-32188 question CBM: minimal handling of certainty -1

Certainty -1 has never been used in standard Moodle, but is
used in Tony-Gardiner Medwin's patches to mean 'No idea' which
we intend to implement: MDL-42077. In the mean time, these changes
avoid errors for people who have used TGM's patches.
MDL-32188 question CBM: use short certainty name in response summaries.