by Tim Hunt.
What is going on is that probably these quizzes were initally created using a restore, and the restore was buggy (there isn't a question 0, so there should never be a reference to one).
Therefore, the DB contained some garbage related to this quiz which did not acutally do any harm. However, as part of the restore into Moodle 2.9.1, we are ignoring that garbage. The warning is just to let you know that has happened.
You may wish to check the restored quiz a bit, to be sure that it is working correctly.
But, if it is working (which is very likely) you can just ignore the warning.
(If is is not working, the warnings might help debug it.)