mail: Fix endless notification email

Yobi was sending a notification email endlessly if it failed to delete
the email from the sending queue. It may happen if the database is
corrupted.

Do not send the notification email if an exception was thrown when
deleting it from the queue(notification_email table).

app/models/NotificationMail.java

@@ -139,10 +139,11 @@ private void sendMail() {
                                     .orderBy("notificationEvent.created ASC").findList();
 
                     for (NotificationMail mail: mails) {
-                        if (mail.notificationEvent.resourceExists()) {
-                            sendNotification(mail.notificationEvent);
-                        }
+                        NotificationEvent event = mail.notificationEvent;
                         mail.delete();
+                        if (event.resourceExists()) {
+                            sendNotification(event);
+                        }
                     }
                 }
             },