Further observations:

parser.add_subparsers() accepts a 'dest' keyword arg, but not a 'required' one.  Default of 'dest' is SUPPRESS, so the name does not appear in the Namespace. Changing it to something like 'command' will produce an entry, e.g. Namespace(command=foo, ...).  Is this a problem?  

Assuming we have a clean way of assigning a name to 'subparsers', what should it be?  'command', '{cmd}', '{foo,bar,baz}' (like in the usage line)?  This name also could be used when telling the user the subparser choice is invalid (parser._check_value).

This issue exposes a problem with '_get_action_name()'.  This function gets a name from the action's option_strings, metavar or dest.  If it can't get a string, it returns None.

ArgumentError pays attention to whether this action name is a string or None, and adjusts its message accordingly.  But the new replacement for the 'too few arguments' error message does a ', '.join([action names]), which chokes if one of those names is None.  There is a mutually_exclusive_groups test that also uses this 'join'.  This bug should be fixed regardless of what is done with subparsers error messages.

So the issues are:
- making 'subparsers' a required argument
- choosing or generating an appropriate name for 'subparsers'
- passing this name to the error message (via _get_action_name?)
- correcting the handling of action names when they are unknown (None).