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()
        object_key = 'data'
        self.s3_client.create_bucket(Bucket=bucket_name)
        body = 'Hello\r\n\r\n\r\n\r\n'
        headers = """
            Authorization: foobar
            Content-Type: audio/mpeg
            X-Amz-Content-Sha256: STREAMING-AWS4-HMAC-SHA256-PAYLOAD
            X-Amz-Date: 20190918T051509Z
            X-Amz-Decoded-Content-Length: %s
        """ % len(body)
        headers = dict([[field.strip() for field in pair.strip().split(':', 1)]
            for pair in headers.strip().split('\n')])
        data = ('d;chunk-signature=af5e6c0a698b0192e9aa5d9083553d4d241d81f69ec62b184d05c509ad5166af\r\n' +
            '%s\r\n0;chunk-signature=f2a50a8c0ad4d212b579c2489c6d122db88d8a0d0b987ea1f3e9d081074a5937\r\n') % body
        
        url = '%s/%s/%s' % (config.TEST_S3_URL, bucket_name, object_key)
        req = PutRequest(url, to_bytes(data), headers)
        urlopen(req, context=ssl.SSLContext()).read()
        
        downloaded_object = self.s3_client.get_object(Bucket=bucket_name, Key=object_key)
        download_file_object = to_str(downloaded_object['Body'].read())
        self.assertEqual(len(str(download_file_object)), len(body))
        self.assertEqual(str(download_file_object), body)
        
        self._delete_bucket(bucket_name, [object_key])
    def test_s3_put_object_on_presigned_url(self):