fix(catalog/rest): Handle decoding errors from non-200 responses

[DerGut]

I ran a simple example without having setup my catalog URI correctly. This code snippet:

catalog, err := rest.NewCatalog(context.TODO(), "test", "http://localhost:8181")
if err != nil {
  slog.Error("Failed to create iceberg catalog", "error", err)
  return
}

errored with

2025/08/10 20:50:39 ERROR Failed to create iceberg catalog error=": "

---

Turns out, when the catalog is returning a non-200 response, we never check for decoding errors which is why we get the unhelpful ": " error.

In this PR, I've refactored the func handleNon200 a little bit to accommodate this. The code is now instead returning something like "decoding json response: EOF: REST error: server error". I have a corresponding commit that shows this new behavior, it's not yet included in this PR because it's a bit clunky.

[zeroshade]

Can we add a test for this please?

[zeroshade]

should probably invert this. First check for the override, and if there's no override then use errFromStatusCode