Allow to customize "missing provider" error URL?
Hi @simov, 

I would like to be able to customize the default response location if a provider hasn't been configured yet. Currently, it is hardcoded to `"/"`:

https://github.com/simov/grant/blob/c220e9ef812978608a4c05551b3f241e3e9da249/lib/response.js#L108

The reasoning is: I'd like to provide a friendly message when a provider wasn't configured, instead of showing this:

![image](https://github.com/simov/grant/assets/130494/580dfbf6-fe2b-496c-968d-0240d8ea76d0)

I'd like to send a PR to allow customizing this, would you mind pointing me a preferred direction to allow such customization?

(Thank you for your work again 🙏  `grant` has been in my toolbelt for years, and it's really great!)