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.
queue_url, queue_attributes = self._create_test_queue()
        self._create_test_notification_bucket(queue_attributes)
        
        file_size = 75000000
        large_file = self.generate_large_file(file_size)
        download_file = new_tmp_file()
        try:
            self.s3_client.upload_file(Bucket=TEST_BUCKET_WITH_NOTIFICATION,
                                       Key=large_file.name, Filename=large_file.name)
            self.assertEqual(self._get_test_queue_message_count(queue_url), '1')
            
            messages = self.sqs_client.receive_message(QueueUrl=queue_url, AttributeNames=['All'])
            message = json.loads(messages['Messages'][0]['Body'])
            self.assertEqual(message['Records'][0]['eventName'], 'ObjectCreated:CompleteMultipartUpload')
            
            self.s3_client.download_file(Bucket=TEST_BUCKET_WITH_NOTIFICATION,
                                         Key=large_file.name, Filename=download_file)
            self.assertEqual(os.path.getsize(download_file), file_size)
            
            self.sqs_client.delete_queue(QueueUrl=queue_url)
            self._delete_bucket(TEST_BUCKET_WITH_NOTIFICATION, large_file.name)
        finally:
            
            large_file.close()
            rm_rf(large_file.name)
            rm_rf(download_file)
    def test_s3_multipart_upload_with_small_single_part(self):