macros should take a status code as their second argument

[yoshuawuyts]

Using ensure! and pals currently always yield a 500 internal server errror. We should make it so that an Into<StatusCode> must always be provided so these actually function as shorthands:

ensure!(tweet.len() < 140, 400, "Tweet must be under 140 characters");

[jbr]

It's not entirely clear what the second arg is when it's just "400," but since it's a macro, what if it took a named argument like an optional status: 400, as in

ensure!(tweet.len() < n, "Tweet must be under {} characters", n, status: 400);

I think that should be possible? We'd just need to distinguish it from the format_args variadic. An upside of this is if it's an optional arg, it's not semver-major.

[yoshuawuyts]

I found jshttp/http-errors recently, which uses a very similar API to the one I proposed:

var err = createError(404, 'This video does not exist!')

I think making the status code always required is not a bad idea, especially since they're convenient to author now that we can convert them from any number. So the equivalent JS code in http-types would look like:

let err = format_err!(404, "This video does not exist");

[yoshuawuyts]

Prior art for for assertions exists in jshttp/http-assert:

assert(username === 'fjodor', 401, 'authentication failed')