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.
return """
            <PostResponse>
                <Location>{protocol}://{host}/{encoded_key}</Location>
                <Bucket>{bucket}</Bucket>
                <Key>{key}</Key>
                <ETag>{etag}</ETag>
            </PostResponse>
            """.format(
            protocol=get_service_protocol(),
            host=config.HOSTNAME_EXTERNAL,
            encoded_key=urlparse.quote(key, safe=''),
            key=key,
            bucket=bucket_name,
            etag='d41d8cd98f00b204e9800998ecf8427f',
        )
    def get_forward_url(self, method, path, data, headers):
        def sub(match):
            
            bucket_name = normalize_bucket_name(match.group(1))
            return '/%s%s' % (bucket_name, match.group(2) or '')
        path_new = re.sub(r'/([^?/]+)([?/].*)?', sub, path)
        if path == path_new:
            return
        url = '%s://%s:%s%s' % (get_service_protocol(), constants.LOCALHOST,
            constants.DEFAULT_PORT_S3_BACKEND, path_new)
        return url
    def return_response(self, method, path, data, headers, response):