Possible fix for issue 25

Not sure if this is the right fix, but I tended to trigger an exception for list indices by running a test script like the one below. Should fail on user auth, but we want to see the reason for failure reported to the user.

```
#!/usr/bin/python -tt
import librato
api = librato.connect("user", "token")
q = api.new_queue()
q.submit()
```