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=73ebb5d7ab95b0547411723351db05cbe628002d
http://github.com/moodle/moodle/commit/73ebb5d7ab95b0547411723351db05cbe628002d
+ 2.1 en [errgradesetanswerblank,qtype_multichoice]