Author: Jerome Mouneyrac
MDL-30459 better error message when file already exist
http://git.moodle.org/gw?p=moodle.git;a=commit;h=f25aee3251b6a55547c5c4da7fa2d0567540b62b
http://github.com/moodle/moodle/commit/f25aee3251b6a55547c5c4da7fa2d0567540b62b
+ 2.2 en [filenameexist,core_webservice]