How to use the honcho.command.CommandError function in honcho

def _procfile(filename):
    try:
        with open(filename) as f:
            content = f.read()
    except IOError:
        raise CommandError('Procfile does not exist or is not a file')

    try:
        procfile = environ.parse_procfile(content)
    except AssertionError as e:
        raise CommandError(str(e))

    return procfile

def _procfile(filename):
    try:
        with open(filename) as f:
            content = f.read()
    except IOError:
        raise CommandError('Procfile does not exist or is not a file')

    try:
        procfile = environ.parse_procfile(content)
    except AssertionError as e:
        raise CommandError(str(e))

    return procfile

def _mkdir(path):
    if os.path.exists(path):
        return
    try:
        os.makedirs(path)
    except OSError as e:
        log.error("Could not create export directory")
        raise CommandError(e)

def command_start(args):
    procfile_path = _procfile_path(args.app_root, _choose_procfile(args))
    procfile = _procfile(procfile_path)

    concurrency = _parse_concurrency(args.concurrency)
    env = _read_env(args.app_root, args.env)
    quiet = _parse_quiet(args.quiet)
    port = _choose_port(args, env)

    if args.processes:
        processes = compat.OrderedDict()
        for name in args.processes:
            try:
                processes[name] = procfile.processes[name]
            except KeyError:
                raise CommandError("Process type '{0}' does not exist in Procfile".format(name))
    else:
        processes = procfile.processes

    manager = Manager(Printer(sys.stdout,
                              colour=(not args.no_colour),
                              prefix=(not args.no_prefix)))

    for p in environ.expand_processes(processes,
                                      concurrency=concurrency,
                                      env=env,
                                      quiet=quiet,
                                      port=port):
        e = os.environ.copy()
        e.update(p.env)
        manager.add_process(p.name, p.cmd, quiet=p.quiet, env=e)