If a multianswer question has its question_multianswer record missing
for some reason, attempting to backup and restore a bank containing the
question will result a dml_missing_record exception.
The qtype_multianswer::get_question_options() method already copes with
the sequence from this record being empty, so we can just remove the
MUST_EXIST check and let it fall back to that.
Mark Johnson
9cd0fe59a3