Author: Andrew Robert Nicols
MDL-32508 Add error when trying to access an unknown course section
http://git.moodle.org/gw?p=moodle.git;a=commit;h=0c053c536821492c2edbec19ea9cc0516b6a72bb
http://github.com/moodle/moodle/commit/0c053c536821492c2edbec19ea9cc0516b6a72bb
+ 2.3 en [unknowncoursesection,core_error]