Author: Tim Hunt
MDL-38279 questions: must have default mark positive.
http://git.moodle.org/gw?p=moodle.git;a=commit;h=00cefd925a421fddb58a021ed8679043c277aaca
http://github.com/moodle/moodle/commit/00cefd925a421fddb58a021ed8679043c277aaca
+ 2.5 en [defaultmarkmustbepositive,core_question]