Author: Mark Nelson
MDL-61255 core_message: add notification when data not migrated
http://git.moodle.org/gw?p=moodle.git;a=commit;h=7022b8d9e4244fb1f2fdce99f5d3f59ce13f7dd2
http://github.com/moodle/moodle/commit/7022b8d9e4244fb1f2fdce99f5d3f59ce13f7dd2
+ 3.5 en [messagingdatahasnotbeenmigrated,core_message]