backend: added warning message when jabberfr workaround is used + fixed missing warning method

src/core/sat_main.py

@@ -37,7 +37,7 @@ from wokkel.xmppim import RosterItem
 
 from sat.bridge.DBus import DBusBridge
 import logging
-from logging import debug, info, error
+from logging import debug, info, warning, error
 
 import sys
 import os.path
@@ -539,6 +539,7 @@ class SAT(service.Service):
         defer_list = []
         for item in disco_result._items:
             if item.entity.full().count('.') == 1: #XXX: workaround for a bug on jabberfr, tmp
+                warning(_('Using jabberfr workaround, be sure your domain has at least two levels (e.g. "example.tld", not "example" alone)'))
                 continue
             args = [item.entity, profile] 
             defer_list.append(disco_client.requestInfo(item.entity).addCallbacks(_check_entity_cb, _errback, args, None, args))