How to use the flintrock.exceptions.ClusterNotFound function in Flintrock

all_clusters_instances = list(
        ec2.instances.filter(
            Filters=[
                {'Name': 'instance-state-name', 'Values': ['pending', 'running', 'stopping', 'stopped']},
                {'Name': 'instance.group-name', 'Values': group_name_filter},
                {'Name': 'vpc-id', 'Values': [vpc_id]},
            ]))

    found_cluster_names = {
        _get_cluster_name(instance) for instance in all_clusters_instances}

    if cluster_names:
        missing_cluster_names = set(cluster_names) - found_cluster_names
        if missing_cluster_names:
            raise ClusterNotFound("No cluster {c} in region {r}.".format(
                c=missing_cluster_names.pop(),
                r=region))

    clusters = [
        _compose_cluster(
            name=cluster_name,
            region=region,
            vpc_id=vpc_id,
            instances=list(filter(
                lambda x: _get_cluster_name(x) == cluster_name, all_clusters_instances)))
        for cluster_name in found_cluster_names]

    return clusters

if not vpc_id:
        vpc_id = get_default_vpc(region=region).id
    else:
        # If it's a non-default VPC -- i.e. the user set it up -- make sure it's
        # configured correctly.
        check_network_config(
            region_name=region,
            vpc_id=vpc_id,
            subnet_id=subnet_id)

    try:
        get_cluster(
            cluster_name=cluster_name,
            region=region,
            vpc_id=vpc_id)
    except ClusterNotFound as e:
        pass
    else:
        raise ClusterAlreadyExists(
            "Cluster {c} already exists in region {r}, VPC {v}.".format(
                c=cluster_name,
                r=region,
                v=vpc_id))

    flintrock_security_groups = get_or_create_flintrock_security_groups(
        cluster_name=cluster_name,
        vpc_id=vpc_id,
        region=region)
    user_security_groups = get_security_groups(
        vpc_id=vpc_id,
        region=region,
        security_group_names=security_groups)