client: Forward skopeo errors - #3172

Yeah, I just spent the past 10-15min seeing if we can do something clean there but I don't think so.

For both OCI and SimpleStreams, we can have a clean authoritative error on the client side, so those would be easy to convert into a generic ErrNoSuchImageAlias.

It's the Incus protocol that's more problematic as over there it returns both 404 or 401/403 if an authorizer is used.

In addition, it's the same GetImageAlias which is used for explicit requests (incus image alias show) as well as indirectly within other logic. The indirect case would benefit from the simple error matching, but the explicit call should return the full server-side message back to the user.

Knowing how LXD/Incus is being used today, we really should have not put so much emphasize on the image fingerprints in the first place and build the entire thing around the aliases instead with specific syntax (sha256:FINGERPRINT type thing) to select a specific image but with the CLI effectively then knowing to use a different code path. Would probably also have made it easier to not get so tied to sha256... Anyway, not going to redo that now :)