title: While using Python's add_argument in argparse, how can I throw an exception if a particular deprecated flag is called?
body: <p>Basically imagine that I have argparser that has multiple arguments.
  I have a particular function definition that looks like this:</p>
  
  <pre><code>    def add_to_parser(self, parser):
          group = parser.add_argument_group('')
          group.add_argument( '--deprecateThis', action='throw exception', help='Stop using this. this is deprecated')
  </code></pre>
  
  <p>Whether I can try and create that action to throw an exception and stop the code or if I can wrap it to check for the <code>deprecateThis</code> flag and then throw an exception, I'd like to know how to do it and which is best!  Thanks.</p>