To have it activated on beta & test wikipedia first before releasing it to the wiki world!
This task is based on a wish that received 21 points (#9) in the German Community Wishlist. Originally, the wish was to receive an information about the sent messages. To help people use mentions in a way that they are sent, we have also included notifications about mention failures, which add the reasons for the failure as well.
 Link to wish (in German): https://de.wikipedia.org/wiki/Wikipedia:Umfragen/Technische_W%C3%BCnsche_2015/Topw%C3%BCnsche#Bei_Speichern_Hinweis_.C3.BCber_abgegebene_Benachrichtigungen_.5BUmfrage_2015.5D
in English: https://meta.wikimedia.org/wiki/WMDE_Technical_Wishes#Notifications_about_failed_mentions