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

[kobajagi]

Legit:

> go run . storage presign predrag-test-bucket/README1.md
⚠️ The object predrag-test-bucket/README1.md does not exist. The Presigned URL will return 404 ⚠️

https://sos-ch-gva-2.exo.io/predrag-test-bucket/README1.md?X-Amz-Algorithm=***....

[imiric]

Thanks, this seems useful. 👍 Just left a minor formatting suggestion.

It would be good to have a test for it, but it's not a blocker.

The main CI workflow didn't run for some reason (I didn't see an approval request), but make test passed locally for me.

[imiric]

Thanks for the changes!