Author: Dongsheng Cai
MDL-69956 forum: throw exception when advanced grading is not ready
http://git.moodle.org/gw?p=moodle.git;a=commit;h=2410df1fd2d4c0ec2fe31a52b7dfd5e4f80026b8
http://github.com/moodle/moodle/commit/2410df1fd2d4c0ec2fe31a52b7dfd5e4f80026b8
+ 3.9 en [error:gradingunavailable,core_grading]