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.
self.s3_client.create_bucket(Bucket=TEST_BUCKET_NAME_WITH_POLICY)
        
        policy = {
            'Version': '2012-10-17',
            'Statement': {
                'Action': ['s3:GetObject'],
                'Effect': 'Allow',
                'Resource': 'arn:aws:s3:::bucketName/*',
                'Principal': {
                    'AWS': ['*']
                }
            }
        }
        response = self.s3_client.put_bucket_policy(
            Bucket=TEST_BUCKET_NAME_WITH_POLICY,
            Policy=json.dumps(policy)
        )
        self.assertEqual(response['ResponseMetadata']['HTTPStatusCode'], 204)
        
        saved_policy = self.s3_client.get_bucket_policy(Bucket=TEST_BUCKET_NAME_WITH_POLICY)['Policy']
        self.assertEqual(json.loads(saved_policy), policy)
    def test_s3_put_object_notification(self):