Give better error messages when client setup fails because of inexistent groups.

Before:

```
Traceback (innermost last):
  Module ZPublisher.Publish, line 126, in publish
  Module ZPublisher.mapply, line 77, in mapply
  Module ZPublisher.Publish, line 46, in call_object
  Module opengever.setup.browser.admin, line 197, in __call__
  Module sqlalchemy.orm.query, line 2028, in __getitem__
IndexError: list index out of range
```

After:
```
Traceback (innermost last):
  Module ZPublisher.Publish, line 126, in publish
  Module ZPublisher.mapply, line 77, in mapply
  Module ZPublisher.Publish, line 46, in call_object
  Module opengever.setup.browser.admin, line 215, in __call__
SetupError: User group 'sdfsdfsdfandant1_users' could not be found.
```