Fix 'ArgumentError' is not defined

To my knowledge, ArgumentError is not a standard python exception but comes from the module argparse. argparse.ArgumentError is for program arguments parsing, not for functions/methods arguments bad values.
In addition, the argparse module is not imported, thus when this line is executed, I get the error: "NameError: name 'ArgumentError' is not defined".

That is why I suggest to raise an Exception object.
Merge pull request #4 from vins31/patch-1

Fix 'ArgumentError' is not defined