[Python] python argparse -- customizing error messages

I want to generate custom error messages for particular usage errors in my command-line program that uses the argparse library. I know I can override the general presentation of the error by subclassing argparse.ArgumentParser:

class HelpParser(argparse.ArgumentParser):
def error(self, message):
sys.stderr.write('error: %s\n' % message)
sys.exit(2)

parser = HelpParser(... ...)
args = parser.parse_args()


But when my error method is called, message has already been formatted by the library. For example,

> python prog.py old stuff

usage: prog [-h] {hot,cold,rain,snow} ...
prog: error: argument subparser: invalid choice: 'old' (choose from u'hot', u'cold', u'rain', u'snow')


How can I change how the stuff after error: is presented, for instance to

usage: prog [-h] {hot,cold,rain,snow} ...
error: 'old' is not a valid option. select from 'hot', 'cold', 'rain', 'snow'


?

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