Surface demo data download error in UI

app.py

@@ -201,11 +201,14 @@ def _ensure_local_file(local_path: Path, hub_filename: str) -> Optional[Path]:
     if local_path.exists() and not _is_git_lfs_pointer(local_path):
         return local_path
 
+    global LAST_DEMO_DOWNLOAD_ERROR
+
     # Prefer a stored token if present (Spaces sometimes have credentials available
     # even when HF_TOKEN env var is not explicitly set).
     token = HF_TOKEN or True
 
     # Try multiple repo types because the artifact may live under a model or dataset repo.
+    last_exc: Exception | None = None
     for repo_type in ("model", "dataset"):
         try:
             cached = hf_hub_download(
@@ -234,6 +237,11 @@ def _ensure_local_file(local_path: Path, hub_filename: str) -> Optional[Path]:
         print(f"[INFO] Using cached Space file for {hub_filename}: {cached_path}")
         return cached_path
     except Exception as exc:
+        # Persist a short error string for the UI status line.
+        err = str(last_exc or exc)
+        if len(err) > 240:
+            err = err[:240] + "..."
+        LAST_DEMO_DOWNLOAD_ERROR = err
         print(
             f"[WARN] Could not download {hub_filename} from Hub ({last_exc}) or Space repo ({exc}); continuing without it."
         )
@@ -241,6 +249,7 @@ def _ensure_local_file(local_path: Path, hub_filename: str) -> Optional[Path]:
 
 
 USING_SYNTHETIC_DATA = False
+LAST_DEMO_DOWNLOAD_ERROR: str | None = None
 
 
 def load_augmented_samples() -> Tuple[List[Dict[str, object]], bool]:
@@ -915,6 +924,9 @@ DATASET_STATUS = (
     f"Demo repo: {HUB_REPO_ID}@{HUB_REVISION or 'main'}"
 )
 
+if LAST_DEMO_DOWNLOAD_ERROR:
+    DATASET_STATUS += f" | Download error: {LAST_DEMO_DOWNLOAD_ERROR}"
+
 has_moe_column = df["moe_embedding"].apply(lambda x: x is not None)
 joint_eval_df = df[has_moe_column & df["joint_label_id"].notna()]