Fix for Issue 1202: missing user name in error message when trying to add inactive user to group

In reference to https://github.com/MarkUsProject/Markus/issues/1202

![screen shot 2013-10-02 at 9 58 40 pm](https://f.cloud.github.com/assets/817212/1258900/a17fe9ce-2bcf-11e3-9c58-636b5168c16f.png)