reduce log level when deliverying notifications to users without permission (#2912)

server/services/notify/notifymentions/mentions_backend.go

@@ -130,8 +130,12 @@ func (b *Backend) BlockChanged(evt notify.BlockChangeEvent) error {
 
 		userID, err := b.deliverMentionNotification(username, extract, evt)
 		if err != nil {
+			if errors.Is(err, ErrMentionPermission) {
+				b.logger.Debug("Cannot deliver notification", mlog.String("user", username), mlog.Err(err))
+			} else {
 				merr.Append(fmt.Errorf("cannot deliver notification for @%s: %w", username, err))
 			}
+		}
 
 		if userID == "" {
 			// was a `@` followed by something other than a username.