Use correct ws on_error type

[liamaharon]

ws on_error panics at the moment because it expects an ErrorEvent with a message but actually receives a generic Event with no message.

https://developer.mozilla.org/en-US/docs/Web/API/WebSocket/error_event#event_type

[aecsocket]

I missed that the event type was an Event not an ErrorEvent there! Since we get a generic Event and have no better message, let's use event.to_string() instead of a hardcoded error message. It probably won't be much more useful, but at least gives a bit of insight into the error.

[liamaharon]

Sure, I've changed it to return .to_string() instead.