Author: David Monllao
MDL-64060 message: Replace error code by language str
http://git.moodle.org/gw?p=moodle.git;a=commit;h=892356acaa195d35750b1ce2b518977db5c9a586
http://github.com/moodle/moodle/commit/892356acaa195d35750b1ce2b518977db5c9a586
+ 3.6 en [errorconversationdoesnotexist,core_message]