Author: Ankit Agarwal
MDL-27884 Implement proper username validation during csv import
http://git.moodle.org/gw?p=moodle.git;a=commit;h=4ad011d041d536d5051b275fdb9dad434816b6bf
http://github.com/moodle/moodle/commit/4ad011d041d536d5051b275fdb9dad434816b6bf
+ 2.2 en [invalidusername,core_error]