Deleting a non-existent key through s3api's delete-objects should return a valid response..

```
aws s3api delete-objects --bucket my-bucket --delete '{\"Objects\":[{\"Key\":\"my-key\"}]}'
```
Gives...
```
{
    "Deleted": [
        {
            "Key": "my-key"
        }
    ]
}
```

Deleting a non-existent key through s3api's delete-objects on LocalStack returns an error response without any additional information beyond the key.

```
aws s3api delete-objects --bucket my-bucket --delete '{"Objects":[{"Key":"my-key"}]}' --endpoint-url http://localhost:4572
```
Gives...
```
{
    "ResponseMetadata": {
        "HTTPStatusCode": 200,
        "HTTPHeaders": {
            "server": "BaseHTTP/0.6 Python/3.7.5",
            "date": "Fri, 20 Dec 2019 16:02:39 GMT",
            "content-type": "application/xml; charset=utf-8",
            "content-length": "180",
            "access-control-allow-origin": "*",
            "last-modified": "Fri, 20 Dec 2019 16:02:39 GMT",
            "access-control-allow-methods": "HEAD,GET,PUT,POST,DELETE,OPTIONS,PATCH",
            "access-control-allow-headers": "authorization,content-type,content-md5,cache-control,x-amz-content-sha256,x-amz-date,x-amz-security-token,x-amz-user-agent,x-amz-target,x-amz-acl,x-amz-version-id,x-localstack-target,x-amz-tagging",
            "access-control-expose-headers": "x-amz-version-id"
        },
        "RetryAttempts": 0
    },
    "Errors": [
        {
            "Key": "my-key"
        }
    ]
}
```

Deleting an existent key seems to work fine.
sqs_client = aws_stack.connect_to_service('sqs')
        try:
            queue_url = queue_url_for_arn(notif['Queue'])
            sqs_client.send_message(QueueUrl=queue_url, MessageBody=message)
        except Exception as e:
            LOGGER.warning('Unable to send notification for S3 bucket "%s" to SQS queue "%s": %s' %
                (bucket_name, notif['Queue'], e))
    if notif.get('Topic'):
        sns_client = aws_stack.connect_to_service('sns')
        try:
            sns_client.publish(TopicArn=notif['Topic'], Message=message, Subject='Amazon S3 Notification')
        except Exception:
            LOGGER.warning('Unable to send notification for S3 bucket "%s" to SNS topic "%s".' %
                (bucket_name, notif['Topic']))
    
    lambda_function_config = notif.get('CloudFunction') or notif.get('LambdaFunction')
    if lambda_function_config:
        
        connection_config = botocore.config.Config(read_timeout=300)
        lambda_client = aws_stack.connect_to_service('lambda', config=connection_config)
        try:
            lambda_client.invoke(FunctionName=lambda_function_config,
                                 InvocationType='Event', Payload=message)
        except Exception:
            LOGGER.warning('Unable to send notification for S3 bucket "%s" to Lambda function "%s".' %
                (bucket_name, lambda_function_config))
    if not filter(lambda x: notif.get(x), NOTIFICATION_DESTINATION_TYPES):
        LOGGER.warning('Neither of %s defined for S3 notification.' %
            '/'.join(NOTIFICATION_DESTINATION_TYPES))
def get_cors(bucket_name):