**Task**: As a user, I want to receive a notification if my mentions were not sent out, because I sent more than 50.
Text for the notification message: <TODO>
**Background**: On the German speaking Community Wishlist, it was proposed to be notified which mentions were sent out (as there are multiple reasons why they sometimes do not get sent out). This proposal received 22 points (#9) in the German Community Wishlist. In addition to successful mentions, we figured that it might also make sense to report back, why unsuccessful mentions were not sent.
 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