Handle invalid schema refs gracefully

[simontaurus]

 200 None
/usr/local/docker-container/package-maintenance/.venv/lib/python3.13/site-packages/osw/wtsite.py:234: RuntimeWarning: WARNING: Page with title 'undefined' does not exist.
  return func(self, *args, **kwargs)
(1/1) Page loaded. Page with title 'undefined' does not exist.
Error: Page undefined does not exist

keeps a "$ref": "/wiki/undefined?action=raw&slot=jsonschema", in the schema but no stub file on disk, leading later to

  File "/usr/local/docker-container/package-maintenance/.venv/lib/python3.13/site-packages/datamodel_code_generator/__init__.py", line 173, in load_data_from_path
    with contextlib.suppress(json.JSONDecodeError), path.open(encoding=encoding) as f:
                                                    ~~~~~~~~~^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.13/pathlib/_local.py", line 539, in open
    return io.open(self, mode, buffering, encoding, errors, newline)
           ~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
FileNotFoundError: [Errno 2] No such file or directory: '/usr/local/docker-container/package-maintenance/.venv/lib/python3.13/site-packages/osw/model/undefined.json'