old_contents: #!/usr/bin/env python3
  
  # chameleon-crawler
  #
  # Copyright 2014 ghostwords.
  #
  # This Source Code Form is subject to the terms of the Mozilla Public
  # License, v. 2.0. If a copy of the MPL was not distributed with this
  # file, You can obtain one at http://mozilla.org/MPL/2.0/.
  
  from argparse import ArgumentParser
  from glob import glob
  from os import path
  
  def is_valid_file(f, parser):
      if path.isfile(f):
          return f
      else:
          return parser.optparser.error("%s does not exist!" % f)
  
  def parse_args():
      parser = ArgumentParser()
  
      parser.add_argument("--non-headless", action="store_true",
              help="do not use a virtual display")
  
      parser.add_argument("--crx", metavar='CRX_FILE_PATH', action="store",
              type=lambda x: is_valid_file(x, parser),
              default=max(glob("*.crx"), key=path.getmtime),
              help="path to Chrome extension CRX package")
  
      return parser.parse_args()
  
new_contents: #!/usr/bin/env python3
  
  # chameleon-crawler
  #
  # Copyright 2014 ghostwords.
  #
  # This Source Code Form is subject to the terms of the Mozilla Public
  # License, v. 2.0. If a copy of the MPL was not distributed with this
  # file, You can obtain one at http://mozilla.org/MPL/2.0/.
  
  from glob import glob
  from os import path
  
  import argparse
  
  def is_valid_file(f, parser):
      if path.isfile(f):
          return f
      raise argparse.ArgumentTypeError("%s does not exist!" % f)
  
  def parse_args():
      parser = argparse.ArgumentParser()
  
      parser.add_argument("--non-headless", action="store_true",
              help="do not use a virtual display")
  
      parser.add_argument("--crx", metavar='CRX_FILE_PATH', action="store",
              type=lambda x: is_valid_file(x, parser),
              default=max(glob("*.crx"), key=path.getmtime),
              help="path to Chrome extension CRX package")
  
      return parser.parse_args()
  
subject: Fix --crx arg error reporting.