Small bugfix in plugins error message

lodel/plugin/plugins.py

         if not(activable is True):
             msg = "Plugin %s is not activable : %s"
             msg %= (self.name, activable)
-            raise PluginError(activable)
+            raise PluginError(msg)
 
         #Circular dependencie detection
         if self.name in self._load_called: