retry when invalid file

pkg/thread.py

@@ -21,9 +21,10 @@ def download_tile(task):
         output_path: Path = output / f"{row}_{col}.png"
         if output_path.exists():
             if not is_png(output_path):
-                # Delete the file and try again later.
+                # Delete the file and try again.
                 del_path(output_path)
-                return row, col, 'failure'
+                # return row, col, 'failure'
+            else:
                 return row, col, 'exist'
         tile_url = f"{base_url}/{row}/{col}"
         response = requests.get(tile_url, headers=r_headers, proxies=PROXIES if use_proxy else None, timeout=60)