If I remove the exit, you get this:

```
Scanning posts.[2014-02-05T19:40:28Z] ERROR: Nikola: You have tags that are too similar: foo and fOo
[2014-02-05T19:40:28Z] ERROR: Nikola: Tag foo is used in: posts/foo.rst
[2014-02-05T19:40:28Z] ERROR: Nikola: Tag fOo is used in: posts/foo.rst
[2014-02-05T19:40:28Z] ERROR: Nikola: You have tags that are too similar: Foo and fOo
[2014-02-05T19:40:28Z] ERROR: Nikola: Tag Foo is used in: posts/foo.rst
[2014-02-05T19:40:28Z] ERROR: Nikola: Tag fOo is used in: posts/foo.rst
...done!
ERROR: Task generation 'render_site' has duplicated definition of 'render_tags:output/categories/foo.xml'
```