Author: Ankit Agarwal
MDL-27884 Implement proper username validation during csv import
http://git.moodle.org/gw?p=moodle.git;a=commit;h=2e505a05d13f3835da9d37e8d75d0843f0cc16c7
http://github.com/moodle/moodle/commit/2e505a05d13f3835da9d37e8d75d0843f0cc16c7
+ 2.3 en [invalidusername,core_error]