MoveData supports a user raising a custom error within a Lightning Flow. To achieve this you will need to create an output-enabled, text collection variable named Errors.

To set the error, you add the error message to the Errors collection.

When a flow finishes it execution with a value in the Errors collection, the notification will fail as demonstrated below:

If you are wishing to gracefully end the execution of a notification, you can use the Break command as noted in the Flow Command Reference. This will ensure the notification is not marked as a failure.