Check and format --server arg

def server_check(arg):
    """Check and format --server arg
    """
    if arg.startswith(('http://', 'https://', 'http+unix://')):
        return arg
    if arg.startswith('./'):
        arg = os.path.abspath(arg)
    elif not arg.startswith('/'):
        raise argparse.ArgumentTypeError(
            'Unix socket path must start with / or ./')
    # assume it is a unix socket
    return 'http+unix://{}'.format(url_escape(arg, ''))