This is a revision of yesterday's patch.  It includes a couple of test cases that check that parser, groups, and exclusive groups all produce the error message.

I also changed the metavar tuple case to return an ArgumentError, and changed test_argparse.py accordingly.  (this metavar testing was added in http://bugs.python.org/issue9348).

This issue overlaps with http://bugs.python.org/issue16970, 'argparse: bad nargs value raises misleading message'.  There, though, the focus is  more on the error message when nargs=0.  While nargs<1 might not make much sense, it technically does not break anything.