Once both success and mention failure notifications are merged, please deploy it to the test cluster for thorough testing
Background:
This task is based on a wish that received 21 points (#9) in the German Community Wishlist (T139962). 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.