How to use the planemo.io.error function in planemo

shed_target_source = kwds.get("shed_target_source")

    label_a = "_%s_" % (shed_target_source if shed_target_source else "workingdir")
    shed_target = kwds.get("shed_target", "B")
    if "/" in shed_target:
        shed_target = "custom_shed"
    label_b = "_%s_" % shed_target

    mine = os.path.join(working, label_a)
    other = os.path.join(working, label_b)

    shed_context = get_shed_context(ctx, read_only=True, **kwds)
    # In order to download the tarball, require repository ID...
    repo_id = realized_repository.find_repository_id(ctx, shed_context)
    if repo_id is None:
        error("shed_diff: Repository [%s] does not exist in the targeted Tool Shed."
              % realized_repository.name)
        # $ diff README.rst not_a_file 2&>1 /dev/null; echo $?
        # 2
        return 2
    info("Diffing repository [%s]" % realized_repository.name)
    download_tarball(
        ctx,
        shed_context,
        realized_repository,
        destination=other,
        clean=True,
        destination_is_pattern=False,
        **kwds
    )
    if shed_target_source:
        new_kwds = kwds.copy()

def download_tarball(ctx, shed_context, realized_repository, **kwds):
    repo_id = realized_repository.find_repository_id(ctx, shed_context)
    if repo_id is None:
        message = "Unable to find repository id, cannot download."
        error(message)
        raise Exception(message)
    destination_pattern = kwds.get('destination', 'shed_download.tar.gz')
    if kwds.get("destination_is_pattern", True):
        destination = realized_repository.pattern_to_file_name(destination_pattern)
    else:
        destination = destination_pattern
    to_directory = not destination.endswith("gz")
    download_tar(shed_context.tsi, repo_id, destination, to_directory=to_directory)
    if to_directory:
        clean = kwds.get("clean", False)
        if clean:
            archival_file = os.path.join(destination, ".hg_archival.txt")
            if os.path.exists(archival_file):
                os.remove(archival_file)

def _create_shed_config(ctx, path, **kwds):
    name = kwds.get("name") or path_to_repo_name(os.path.dirname(path))
    name_invalid = validate_repo_name(name)
    if name_invalid:
        error(name_invalid)
        return 1

    owner = kwds.get("owner")
    if owner is None:
        owner = ctx.global_config.get("shed_username")
    owner_invalid = validate_repo_owner(owner)
    if owner_invalid:
        error(owner_invalid)
        return 1
    description = kwds.get("description") or name
    long_description = kwds.get("long_description")
    remote_repository_url = kwds.get("remote_repository_url")
    homepage_url = kwds.get("homepage_url")
    categories = kwds.get("category", [])
    config = dict(
        name=name,

def not_specifing_dependent_option(x, y):
        if kwds.get(x) and not kwds.get(y):
            template = "Can only use the --%s option if also specifying --%s"
            message = template % (x, y)
            io.error(message)
            return True

def not_specifing_dependent_option(x, y):
        if kwds.get(x) and not kwds.get(y):
            template = "Can only use the --%s option if also specifying --%s"
            message = template % (x, y)
            io.error(message)
            return True

condarc_override = kwds.get("condarc", condarc_override_default)
    use_local = kwds.get("conda_use_local", False)
    shell_exec = shell if use_planemo_shell else None
    conda_context = conda_util.CondaContext(conda_prefix=conda_prefix,
                                            ensure_channels=ensure_channels,
                                            condarc_override=condarc_override,
                                            use_local=use_local,
                                            shell_exec=shell_exec)
    handle_auto_init = kwds.get("handle_auto_init", False)
    if handle_auto_init and not conda_context.is_installed():
        auto_init = kwds.get("conda_auto_init", True)
        failed = True
        if auto_init:
            if conda_context.can_install_conda():
                if conda_util.install_conda(conda_context):
                    error(MESSAGE_ERROR_FAILED_INSTALL)
                else:
                    failed = False
            else:
                error(MESSAGE_ERROR_CANNOT_INSTALL)
        else:
            error(MESSAGE_ERROR_NOT_INSTALLING)

        if failed:
            raise ExitCodeException(EXIT_CODE_FAILED_DEPENDENCIES)
    if handle_auto_init:
        conda_context.ensure_conda_build_installed_if_needed()
    return conda_context

def name_to_command(name):
    """Convert a subcommand name to the cli function for that command.

    Command  is defined by the method 'planemo.commands.cmd_:cli',
    this method uses `__import__` to load and return that method.
    """
    try:
        if sys.version_info[0] == 2:
            name = name.encode('ascii', 'replace')
        mod_name = 'planemo.commands.cmd_' + name
        mod = __import__(mod_name, None, None, ['cli'])
    except ImportError as e:
        error("Problem loading command %s, exception %s" % (name, e))
        return
    return mod.cli

def _create_shed_config(ctx, path, **kwds):
    name = kwds.get("name") or path_to_repo_name(os.path.dirname(path))
    name_invalid = validate_repo_name(name)
    if name_invalid:
        error(name_invalid)
        return 1

    owner = kwds.get("owner")
    if owner is None:
        owner = ctx.global_config.get("shed_username")
    owner_invalid = validate_repo_owner(owner)
    if owner_invalid:
        error(owner_invalid)
        return 1
    description = kwds.get("description") or name
    long_description = kwds.get("long_description")
    remote_repository_url = kwds.get("remote_repository_url")
    homepage_url = kwds.get("homepage_url")
    categories = kwds.get("category", [])
    config = dict(
        name=name,
        owner=owner,
        description=description,
        long_description=long_description,
        remote_repository_url=remote_repository_url,
        homepage_url=homepage_url,
        categories=categories,
    )
    # Remove empty entries...