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()
        self.s3_client.create_bucket(Bucket=bucket_name)
        
        object_key = 'key-by-hostname'
        self.s3_client.put_object(Bucket=bucket_name, Key=object_key, Body='something')
        
        url = self.s3_client.generate_presigned_url(
            'get_object', Params={
                'Bucket': bucket_name,
                'Key': object_key,
                'ResponseCacheControl': 'max-age=74',
                'ResponseContentDisposition': 'attachment; filename="foo.jpg"',
                'ResponseContentEncoding': 'identity',
                'ResponseContentLanguage': 'de-DE',
                'ResponseContentType': 'image/jpeg',
                'ResponseExpires': 'Wed, 21 Oct 2015 07:28:00 GMT'}
        )
        response = requests.get(url, verify=False)
        self.assertEqual(response.headers['cache-control'], 'max-age=74')
        self.assertEqual(response.headers['content-disposition'], 'attachment; filename="foo.jpg"')
        self.assertEqual(response.headers['content-encoding'], 'identity')
        self.assertEqual(response.headers['content-language'], 'de-DE')
        self.assertEqual(response.headers['content-type'], 'image/jpeg')
        self.assertEqual(response.headers['expires'], '2015-10-21T07:28:00Z')
        
        self._delete_bucket(bucket_name, [object_key])
    def test_s3_copy_md5(self):