Clarify transient training progress upload errors

tools/windows_training_agent.py

@@ -237,10 +237,16 @@ def safe_report_progress(
     message: str,
     log_path: Path,
 ) -> None:
+    last_error: Exception | None = None
+    for attempt in range(1, 4):
         try:
             report_progress(args, job_id, status, phase, progress_percent, message)
+            return
         except Exception as exc:  # noqa: BLE001 - keep the local training process alive.
-        log(log_path, f"Progress report failed: {exc}")
+            last_error = exc
+            if attempt < 3:
+                time.sleep(attempt * 2)
+    log(log_path, f"Temporary progress upload error; training continues: {last_error}")
 
 
 def api_json(args: argparse.Namespace, path: str, payload: dict[str, Any], timeout: int = 30) -> dict[str, Any]: