`NoContent` is not supported when using `Cookie` / `JWT` + custom monad

[dcastro]

This is a follow-up to: #1812

So to summarize for future encounterers of this error message: you get this error when you have any route in your API where a handler returns NoContent when you try to wrap the API with cookies.

I'm opening this issue because 2 of his points are worth addressing

I think perhaps Servant should give a more easily digestible error message here -- since we are already in compilation failure territory and the application cannot be built, it should be possible to change the specifics of the failure while still preserving it as a compiler error. Could Servant define an instance covering this situation, but with a more helpful error message for newcomers?

I think at least we need to make it much more explicit in the documentation that using Cookie, or anything that introduces Cookie, like JWT, means you must eliminate NoContent responses from your API.