Low level issue if no server nor auth is passed

```
$ kinto-wizard load /tmp/test.yaml
Traceback (most recent call last):
File "/home/mathieu/Code/Mozilla/kinto-wizard/.venv/bin/kinto-wizard", line 11, in
load_entry_point('kinto-wizard', 'console_scripts', 'kinto-wizard')()
File "/home/mathieu/Code/Mozilla/kinto-wizard/kinto_wizard/__main__.py", line 60, in main
client = cli_utils.create_client_from_args(args)
File "/home/mathieu/Code/Mozilla/kinto-wizard/.venv/local/lib/python3.6/site-packages/kinto_http/cli_utils.py", line 28, in create_client_from_args
ignore_batch_4xx=args.ignore_batch_4xx)
File "/home/mathieu/Code/Mozilla/kinto-wizard/.venv/local/lib/python3.6/site-packages/kinto_http/__init__.py", line 75, in __init__
self.session = create_session(**session_kwargs)
File "/home/mathieu/Code/Mozilla/kinto-wizard/.venv/local/lib/python3.6/site-packages/kinto_http/session.py", line 40, in create_session
raise AttributeError(msg)
AttributeError: You need to either set session or auth + server_url

```

Reproduce `kinto-wizard load /tmp/test.yaml` without server or auth, then inspect `kinto_wizard/__main__.py` around `main()` and the `create_client_from_args` call shown in the traceback. The issue does not define the intended user-facing behavior; clarify that first, then ensure the missing configuration no longer surfaces only as the low-level `AttributeError`.

Written by the indexing model from the issue text.