This patch addresses both issues raised here:
- throw an error when the subparser argument is missing
- allow the subparser argument to be optional

argparse.py:

_SubParsersAction -
   add 'required=True' keyword.

   name(self) method - creates a name of the form {cmd1,cmd2} for error messages.

_get_action_name() - try action.name() if it can't get a name from option_strings, dest or metavar.  Still can return None.

2 error cases do a join on a list of action_names.  If a name is None, this will choke.  Add a ['%s'%x for x in list] guard.

test_argparse.py: 
add cases to the subparser block to test the 'required' keyword, and to test the error message changes.

argparse.rst:
add a summary of the add_subparsers() arguments.