Author: Andrew Robert Nicols
MDL-29818: Improve error message when a grade has no answer
http://git.moodle.org/gw?p=moodle.git;a=commit;h=5a6084cf61d87a9d34d302017777c51e0bb0db46
http://github.com/moodle/moodle/commit/5a6084cf61d87a9d34d302017777c51e0bb0db46
+ 2.2 en [errgradesetanswerblank,qtype_multichoice]