disable plugin correctly when not configured

nospam/plugins/google_sb.py

@@ -12,8 +12,7 @@ class GoogleSafeBrowsingPlugin(BasePlugin):
     def __init__(self, config):
         api_key = config.get('safebrowsing_api_key')
         if not api_key:
-            self.enabled = False
-            return
+            raise Exception('Google SafeBrowsing API key not configured')
         data_dir = config.get('data_dir', './data')
         dspath = os.path.join(data_dir, 'nospam_gsb_datastore')
         ds = datastore.DataStore(dspath)