Also change the icon in case of network error on update

mitterlib/ui/ui_pygtk.py

@@ -1028,6 +1028,8 @@ class Interface(object):
         error_win.run()
         error_win.hide()
         self._update_statusbar('Auto-update disabled')
+        if self._statusicon:
+            self._statusicon.set_from_file(find_image('mitter-error.png'))
         return
 
     ### image download function