Problem: Mentions are not always sent out (e.g. if more than 50 people were mentioned at once). Currently, users do not get any feedback if their intended mention failed to send.
Add a new checkbox to the notification settings to opt in to mention failures notifications. If set, users should receive a notification whenever a mention has not been sent out.
N.b.: This is not about checking whether a mention has been delivered, but only if it has been sent out.
Background: 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 keep the number of notifications smaller, and to help people use mentions in a way that they are sent, the task informs about mention failures, and adds 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
Sites that currently describe mention behavior: