Use Stdlib.raise and Lwt.reraise instead of Lwt.fail for better backtraces

[ferminr]

Lwt's documentation reads:

Whenever possible, it is recommended to use raise exn instead, as
raise captures a backtrace, while Lwt.fail does not. If you call
raise exn in a callback that is expected by Lwt to return a
promise, Lwt will automatically wrap exn in a rejected promise,
but the backtrace will have been recorded by the OCaml runtime.

For example, bind's second argument is a callback which returns a
promise. And so it is recommended to use raise in the body of that
callback.

Use Lwt.fail only when you specifically want to create a rejected
promise, to pass to another function, or store in a data structure.

See also mirage/ocaml-cohttp#1079

Note: CI failures on OCaml 5 switches are unrelated to the changes in this PR. That issue is addressed in a separate PR: #35

[raphael-proust]

Suggested change

	| `Exn (Lwt.Canceled as exn) -> log #info "canceling accept loop"; raise exn
	| `Exn (Lwt.Canceled as exn) -> log #info "canceling accept loop"; Lwt.reraise exn

The exn is captured by Exn_lwt.map which should preserve the backtrace of exn. Using reraise here will add to that backtrace rather than starting a new one.

---

You could also write this function directly with Lwt.try_bind (which makes it more obvious where Canceled came from) but that's somewhat orthogonal:

Lwt.try_bind (fun () -> Lwt_unix.accet fd)
  (fun (fd,addr as peer) -> …)
  (function
    | Unix.Unix_error (Unix.EMFILE,_,_) -> …
    | Lwt.Canceled as exn -> …
    | exn -> …)

[ferminr]

Good catch. (Force pushed it)

[raphael-proust]

Apart from the small suggestion above, LGTM