Author: Ankit Agarwal
MDL-27884 Implement proper username validation during csv import
http://git.moodle.org/gw?p=moodle.git;a=commit;h=363ed37e760627bf504766377c75cba90535fa94
http://github.com/moodle/moodle/commit/363ed37e760627bf504766377c75cba90535fa94
+ 2.4 en [invalidusername,core_error]