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.
import re
import uuid
import xmltodict
from moto.sqs.utils import parse_message_attributes
from moto.sqs.models import Message, TRANSPORT_TYPE_ENCODINGS
from six.moves.urllib import parse as urlparse
from six.moves.urllib.parse import urlencode
from requests.models import Request, Response
from localstack import config
from localstack.config import HOSTNAME_EXTERNAL, SQS_PORT_EXTERNAL
from localstack.utils.common import to_str, md5, clone
from localstack.utils.analytics import event_publisher
from localstack.services.awslambda import lambda_api
from localstack.utils.aws.aws_stack import extract_region_from_auth_header
from localstack.services.generic_proxy import ProxyListener
XMLNS_SQS = 'http://queue.amazonaws.com/doc/2012-11-05/'
SUCCESSFUL_SEND_MESSAGE_XML_TEMPLATE = """
    <?xml version="1.0"?>
    <SendMessageResponse xmlns="%s">
        <SendMessageResult>
            <MD5OfMessageAttributes>{message_attr_hash}</MD5OfMessageAttributes>
            <MD5OfMessageBody>{message_body_hash}</MD5OfMessageBody>
            <MessageId>{message_id}</MessageId>
        </SendMessageResult>
        <ResponseMetadata>
            <RequestId>00000000-0000-0000-0000-000000000000</RequestId>
        </ResponseMetadata>