Add explicit discussion of teaching demo 'failure'

This comes up a few times a year on Slack as new Trainers begin hosting their first demos. Should be addressed more thoroughly in Trainer Training.

Start by reviewing the Trainer Training materials to find where teaching-demo guidance is documented. Add an explicit discussion of demo “failure” for new Trainers, including the situations that currently come up on Slack; the relevant training guidance should clearly address this topic.

Written by the indexing model from the issue text.