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.
path = to_str(path)
        method = to_str(method)
        bucket_name = get_bucket_name(path, headers)
        
        hostname_parts = headers['host'].split('.')
        if (not bucket_name or len(bucket_name) == 0) and len(hostname_parts) > 1:
            bucket_name = hostname_parts[0]
        
        
        
        key = None
        if method == 'POST':
            key, redirect_url = multipart_content.find_multipart_key_value(data, headers)
            if key and redirect_url:
                response.status_code = 303
                response.headers['Location'] = expand_redirect_url(redirect_url, key, bucket_name)
                LOGGER.debug('S3 POST {} to {}'.format(response.status_code, response.headers['Location']))
            key, status_code = multipart_content.find_multipart_key_value(
                data, headers, 'success_action_status')
            if response.status_code == 200 and status_code == '201' and key:
                response.status_code = 201
                response._content = self.get_201_reponse(key, bucket_name)
                response.headers['Content-Length'] = str(len(response._content))
                response.headers['Content-Type'] = 'application/xml; charset=utf-8'
                return response
        parsed = urlparse.urlparse(path)
        bucket_name_in_host = headers['host'].startswith(bucket_name)