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.
bucket_name = 'test-bucket-%s' % short_uid()
        key = 'test.file'
        hostname_before = config.HOSTNAME_EXTERNAL
        config.HOSTNAME_EXTERNAL = 'foobar'
        try:
            content = 'test content 123'
            acl = 'public-read'
            self.s3_client.create_bucket(Bucket=bucket_name)
            
            response = self._perform_multipart_upload(bucket=bucket_name, key=key, data=content, acl=acl)
            expected_url = '%s://%s:%s/%s/%s' % (get_service_protocol(), config.HOSTNAME_EXTERNAL,
                config.PORT_S3, bucket_name, key)
            self.assertEqual(expected_url, response['Location'])
            
            self.s3_client.put_object_acl(Bucket=bucket_name, Key=key, ACL=acl)
            
            downloaded_object = self.s3_client.get_object(Bucket=bucket_name, Key=key)
            self.assertEqual(to_str(downloaded_object['Body'].read()), content)
            
            download_url = response['Location'].replace('%s:' % config.HOSTNAME_EXTERNAL, 'localhost:')
            response = safe_requests.get(download_url)
            self.assertEqual(response.status_code, 200)
            self.assertEqual(to_str(response.content), content)
        finally:
            config.HOSTNAME_EXTERNAL = hostname_before
    def test_s3_website_errordocument(self):