This patch adds a value test for nargs during add_argument.  The core of the new test is in ArgumentParser._check_argument.

add_argument() now calls ArgumentParser._check_argument(), which calls _format_args().  If it gets a TypeError, it raises a metavar ValueError as before.  But if it gets an ValueError, it raises an ArgumentError.

An argument group gets the _check_argument method from its container.  This way, check_argument() works for both parsers and groups.

HelpFormater _format_args() now raises a ValueError if the nargs is not an integer (or one of the recognized strings). 

What kind of error should we produce when there is a problem with nargs?

An ArgumentError has the advantage that it includes the action name.  But the metavar tuple test was coded to raise ValueError.  Plus the test_argparse.py TestInvalidArgumentConstructors class tests all check for TypeError or ValueError.

I have kept the metavar tuple case as ValueError.  That way, test_argparse.py runs without a change.  I still need to add tests for invalid string nargs values.  And I think the case could be made for returning ArgumentValue errors.