fix(settings): a catalog fetch that fails logs one line, not a traceback every retry (#5359)

The plugin_catalog_refresh tick retries a failed repository every 5 minutes;
the configured index URL answers 404, which logged a full traceback each time.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

steward/core/plugin_index.py

@@ -75,8 +75,10 @@ async def _fetch_one(url: str) -> bool:
             resp = await client.get(url)
             resp.raise_for_status()
         raw = yaml.safe_load(resp.text) or {}
-    except Exception:
+    except Exception as exc:
-        logger.exception("Failed to fetch plugin catalog from %s", url)
+        # The tick retries every RETRY_AFTER seconds, so a repository that
+        # answers 404 or is unreachable is one line each time, not a traceback.
+        logger.warning("Plugin catalog not fetched from %s: %s", url, exc)
         _failed_at[url] = time.monotonic()
         return False
     _url_cache[url] = (raw, time.monotonic())