AWS Firehose Lambda Transformation Handle errors

Question

I have built a Kinesis Firehose delivery stream that has a lambda function transformation. In that lambda function I loop through my records similar to here. My function works fine, however I have some extra logic that has a throws an error some data is not validated.

The error I get is that Firehose writes to the s3 bucket for the error folder fine, but it does not give me my custom error message only a message indicating that an error occurred within the function. I found this documentation regarding Data Transformation Failure Handling. However it does not tell me how to modify the output.

def lambda_handler(event, context):
    output = []
    try:
        for record in event['records']:
            payload = base64.b64decode(record['data']).decode('utf-8')
            # some other validation logic here
            output_record = {
                'recordId': record['recordId'],
                'result': 'Ok',
                'data': base64.b64encode(payload.encode('utf-8')).decode('utf-8')
            }
            output.append(output_record)
    except Exception as e:
        raise e

    return {'records': output}