chore: expose DWH preprocessing failure detail under P2 debug

harness/tht/cli/preprocess_cmd.py

@@ -4,6 +4,7 @@ from __future__ import annotations
 
 import hashlib
 import json
+import os
 import re
 from pathlib import Path
 
@@ -259,8 +260,10 @@ def dwh_cmd(
         raise typer.Exit(code=2)
     try:
         result = run_dwh_from_config(config, steps=selected, resume=resume)
-    except Exception:  # noqa: BLE001
+    except Exception as exc:  # noqa: BLE001
         payload = {"status": "failed", "error": "DWH preprocessing failed"}
+        if os.environ.get("THT_P2_DEBUG") == "1":
+            payload["error"] = f"DWH preprocessing failed: {type(exc).__name__}: {exc}"
         if json_output:
             typer.echo(json.dumps(payload, sort_keys=True))
         else: