code: def errorhandler(self, code_or_exception):
          
          def decorator(fn):
              self._defer(lambda bp: bp.register_error_handler(code_or_exception, fn))
              return fn
          return decorator
docs: Registers an error handler that becomes active for this bundle
          only.  Please be aware that routing does not happen local to a
          bundle so an error handler for 404 usually is not handled by
          a bundle unless it is caused inside a view function.  Another
          special case is the 500 internal server error which is always looked
          up from the application.
  
          Otherwise works as the :meth:`flask.Blueprint.errorhandler` decorator.