How to use the routemaster.state_machine.exceptions.UnknownLabel function in routemaster

def get_label_metadata(app: App, label: LabelRef) -> Metadata:
    """Returns the metadata associated with a label."""
    state_machine = get_state_machine(app, label)

    row = get_label_metadata_internal(app, label, state_machine)

    if row is None:
        raise UnknownLabel(label)

    metadata, deleted = row

    if deleted:
        raise DeletedLabel(label)

    return metadata

def get_current_history(app: App, label: LabelRef) -> History:
    """Get a label's last history entry."""
    history_entry = app.session.query(History).filter_by(
        label_name=label.name,
        label_state_machine=label.state_machine,
    ).order_by(
        # Our model type stubs define the `id` attribute as `int`, yet
        # sqlalchemy actually allows the attribute to be used for ordering like
        # this; ignore the type check here specifically rather than complicate
        # our type definitions.
        History.id.desc(),  # type: ignore
    ).first()

    if history_entry is None:
        raise UnknownLabel(label)

    return history_entry

"""State machine exceptions."""


class UnknownLabel(ValueError):
    """Represents a label unknown in the given state machine."""
    deleted = False


class DeletedLabel(UnknownLabel):
    """Represents a label deleted in the given state machine."""
    deleted = True


class UnknownStateMachine(ValueError):
    """Represents a state machine not in the system."""


class LabelAlreadyExists(ValueError):
    """Thrown when a label already exists in the state machine."""

def lock_label(app: App, label: LabelRef) -> Label:
    """Lock a label in the current transaction."""
    row = app.session.query(Label).filter_by(
        name=label.name,
        state_machine=label.state_machine,
    ).with_for_update().first()

    if row is None:
        raise UnknownLabel(label)

    return row