docs: def handle_http_error(self, code, error):
          """Helper that renders `error{code}.html`.
  
          Convenient way to use it::
  
             from functools import partial
             handler = partial(app.handle_http_error, code)
             app.errorhandler(code)(handler)
          """
          # 5xx code: error on server side
          if (code // 100) == 5:
              # ensure rollback if needed, else error page may
              # have an error, too, resulting in raw 500 page :-(
              db.session.rollback()
  
          template = f"error{code:d}.html"
          return render_template(template, error=error), code