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.
"""Add Last-Modified header with current time
    (if the response content is an XML containing <LastModified>, add that instead)"""
    time_format = '%a, %d %b %Y %H:%M:%S GMT'  
    try:
        if content:
            last_modified_str = re.findall(r'<LastModified>([^<]*)</LastModified>', content)
            if last_modified_str:
                last_modified_str = last_modified_str[0]
                last_modified_time_format = dateutil.parser.parse(last_modified_str).strftime(time_format)
                response.headers['Last-Modified'] = last_modified_time_format
    except TypeError as err:
        LOGGER.debug('No parsable content: %s' % err)
    except ValueError as err:
        LOGGER.error('Failed to parse LastModified: %s' % err)
    except Exception as err:
        LOGGER.error('Caught generic exception (parsing LastModified): %s' % err)
    
    try:
        if response.headers.get('Last-Modified', '') == '':
            response.headers['Last-Modified'] = datetime.datetime.now().strftime(time_format)
    except Exception as err:
        LOGGER.error('Caught generic exception (setting LastModified header): %s' % err)
def append_list_objects_marker(method, path, data, response):