fix typo in site.py, self.error(404) is valid, not self.not_found(404)

This minor typo breaks multiple failed lookups in the handler, so if you have:
/services/real_slug/fake_foobar [404]
/services/fake_slug/fake_foobar [500]

Since there's no 500 handler, you get an appengine 500 which is tacky.