Update error message when deleting a registered cluster

Change-Id: Idd0017d76127ac2b14cc1d11f2c9cf2ecc1de896
Signed-off-by: garciadeblas <gerardo.garciadeblas@telefonica.com>
diff --git a/osm_nbi/k8s_topics.py b/osm_nbi/k8s_topics.py
index c930472..6e7fd25 100644
--- a/osm_nbi/k8s_topics.py
+++ b/osm_nbi/k8s_topics.py
@@ -565,7 +565,7 @@
         check = self.db.get_one(self.topic, filter_q)
         if check["created"] == "false":
             raise EngineException(
-                "Cannot delete registered cluster",
+                "Cannot delete registered cluster. Please deregister.",
                 HTTPStatus.UNPROCESSABLE_ENTITY,
             )
         super().delete(session, _id, dry_run=False, not_send_msg=None)