Add CLI default help message

[ariel-m-s]

I think that showing a help message when using the CLI in a wrong way would be helpful. For instance, when typing zum (without any arguments), the CLI currently outputs an exception (as shown bellow). I find this to be a little aggresive for newcomers (like me) who are beginning to experiment with Zum.

Traceback (most recent call last):
  File "/usr/local/bin/zum", line 8, in <module>
    sys.exit(dispatcher())
  File "/usr/local/lib/python3.9/site-packages/zum/cli.py", line 22, in dispatcher
    engine.execute(parsed_args.action[0], parsed_args.params)  # pragma: nocover
AttributeError: 'Namespace' object has no attribute 'action'

This help message should’t be as complete as the documentation, but should provide some practical information such as a list of commands and possible arguments.

[ariel-m-s]

I realized that you can type zum --help to get some help. Maybe this message should be extended (with the list of possible commands and that kind of things) and set as a default message everytime the user does something wrong.

[daleal]

The --help message already includes the list of possible commands (when a valid zum.toml file exists), but I do agree that maybe exceptions that aren't caught and stored by the engine should be caught at the CLI level, the exception should be displayed and then the help message should be shown.