docs: def wrap_errorhandler(app):
          """Wrap error handler.
  
          :param app: The Flask application.
          """
          try:
              existing_handler = app.error_handler_spec[None][404][NotFound]
          except (KeyError, TypeError):
              existing_handler = None
  
          if existing_handler:
              app.error_handler_spec[None][404][NotFound] = \
                  lambda error: handle_not_found(error, wrapped=existing_handler)
          else:
              app.error_handler_spec.setdefault(None, {}).setdefault(404, {})
              app.error_handler_spec[None][404][NotFound] = handle_not_found