I'm going to close this.

Python makes it easy to test for Exception class.  Testing exception messages is messy, especially if the test wants to compare the message against 'generic message'.

I don't see anything generic about the messages produced by `int('1.23')` and `float('xxx')`.

Argparse provides a custom class, ArgumentTypeError, for use when you want to pass a custom message.  Let's leave it at that, and not try to make things more complicated.

If there's to be any change it should be in the documentation as suggested in https://bugs.python.org/issue20039.