def update_mode(arg_namespace):
    """Check command line arguments and run update function."""
    try:
        updater.update(custom_sources=arg_namespace.custom)
    except (PermissionError, FileNotFoundError) as exception:
        if isinstance(exception, PermissionError):
            print('No write permission for current working directory.')
        if isinstance(exception, FileNotFoundError):
            print('Necessary resources for updating not found in current '
                  'working directory.')
Check command line arguments and run update function.