Change default error code from 500 to 503 in case message is not ready

Change-Id: Idf80ca72948c8298ba0789d48b1aff941b626bb3
Signed-off-by: tierno <alfonso.tiernosepulveda@telefonica.com>
diff --git a/osm_common/msgbase.py b/osm_common/msgbase.py
index 98f97bf..2b338df 100644
--- a/osm_common/msgbase.py
+++ b/osm_common/msgbase.py
@@ -10,7 +10,7 @@
     Base Exception class for all msgXXXX exceptions
     """
 
-    def __init__(self, message, http_code=HTTPStatus.INTERNAL_SERVER_ERROR):
+    def __init__(self, message, http_code=HTTPStatus.SERVICE_UNAVAILABLE):
         """
         General exception
         :param message:  descriptive text