Information when a unit type does not exist

[elinw]

Currently when running lesson_to_html() a unit type (class) that does not exist results in the processing stopping completely and a somewhat hard to understand message (especially for people who haven't looked at the Swirlify source). This change will simply put the message into the generated html while allowing processing to continue.

[elinw]

Alternatively, the message in stop() could be changed to something a bit more understandable and formatted in a more readable way.
It took me a while to figure out what the message meant, so I thought I would suggest a way of making things a bit more clear.

[seankross]

I think this is a good idea, especially as new question types are developed. Thank you!