May be a duplicate of https://github.com/localstack/localstack/issues/988 but due to the age I will re raise.

I'm having issues when I throw an exception in a lambda - it isn't appearing on my dead letter queue. Is there something I am missing, I would like an sqs event exception to appear on the poison queue when an exception happens.

Below is the basic config:
 ````
    sqsclient.create_queue(QueueName = 'test-queue')
    sqsclient.create_queue(QueueName = 'test-queue-poison')

    lambda_client.create_function(
            FunctionName='MyFunction',
            Runtime='dotnetcore2.1',
            Role='role',
            Handler='Handler',
            Code=code,
	    DeadLetterConfig={'TargetArn': 'test-queue-poison-arn'})

     lambda_client.create_event_source_mapping(
		EventSourceArn='test-queue-arn',
		FunctionName='MyFunction',
		Enabled=True,
		BatchSize=100,
		StartingPosition='LATEST')
````

So when I raise a new (.net)exception within the lambda function no message is going to the dead letter queue. I can confirm that all is attached together OK as all other scenarios work, and the above code is pseudo-ish.
if isinstance(e, ClientError):
            return e.get_response()
        return error_response('Unknown error: %s %s' % (e, traceback.format_exc()))
@app.route('%s/functions/<function>' % PATH_ROOT, methods=['GET'])
def get_function(function):
    """ Get details for a single function
        ---
        operationId: 'getFunction'
        parameters:
            - name: 'request'
              in: body
            - name: 'function'
              in: path
    """
    funcs = do_list_functions()
    for func in funcs:
        if func['FunctionName'] == function:
            result = {
                'Configuration': func,
                'Code': {
                    'Location': '%s/code' % request.url
                }
            }
            lambda_details = arn_to_lambda.get(func['FunctionArn'])
            if lambda_details.concurrency is not None:
                result['Concurrency'] = lambda_details.concurrency
            return jsonify(result)
    return not_found_error(func_arn(function))
@app.route('%s/functions/' % PATH_ROOT, methods=['GET'])
def list_functions():