RemoteGraphClient should raise an error when the URL is wrong

We should try to retrieve the stats endpoint and error if it returns a 404.

added activity::Implementation label

assigned to @lunar

mentioned in issue swh-alter#13 (closed)

changed milestone to %Tooling for takedown notices [Roadmap - Collect]

mentioned in merge request !338 (merged)

closed with merge request !338 (merged)

closed with commit 0bf1c826

mentioned in commit 0bf1c826