How to handle errors in custom formatter parse method (doc bug)?

[KubaO]

The documentation example implies that there's format_parse_context::on_error method. But there doesn't seem to be so the below does not compile:

  constexpr auto parse(format_parse_context& ctx) -> format_parse_context::iterator {
    /* ... */
    if (it != end && *it != '}') ctx.on_error("invalid format");
    /* ... */
  }

[vitaut]

Duplicate of #3458.