feat: add a warning in case the object doesn't exists before generating the presign url

[mlec1]

Description

It happened sometimes that when I want to generate a presigned url, I make a typo in the object name / path.

And it is only when I sent the link to someone or when I want to download the object, that I notice that the URL returns 404. I would prefer to notice it when I generate the URL.

This MR adds a warning message when the object doesn't exists. For the moment, only a warning, I could return the error and no presigned url at all if you prefer.

Checklist

(For exoscale contributors)

• [ ] Changelog updated (under Unreleased block)
• [ ] Testing

Testing