[auto_backup] FIX: failure type notification (#770)

Fixes https://github.com/OCA/server-tools/issues/710

auto_backup/models/db_backup.py

@@ -205,7 +205,10 @@ class DbBackup(models.Model):
                 "<p>%s</p><pre>%s</pre>" % (
                     _("Database backup failed."),
                     escaped_tb),
-                subtype=self.env.ref("auto_backup.failure"))
+                subtype=self.env.ref(
+                    "auto_backup.mail_message_subtype_failure"
+                ),
+            )
         else:
             _logger.info("Database backup succeeded: %s", self.name)
             self.message_post(_("Database backup succeeded."))