fix error message

src/model/gnss.py

@@ -200,7 +200,7 @@ class GNSSCampaign(Object):
             self._cov_mat = None
         except ValueError:
             logger.warn('Station {} does not exist in campaign, '
-                        'do nothing.' % station_code)
+                        'do nothing.'.format(station_code))
 
     def get_station(self, station_code):
         for sta in self.stations: