Use failwith instead of Lwt.fail_with

Lwt's documentation reads:

> In most cases, it is better to use `failwith s` from the standard
> library.

and

> 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.

Prefer to capture backtraces to improve debugability.

lib/btrfs_store.ml

@@ -90,11 +90,8 @@ let check_kernel_version () =
  | Some maj, Some min when (maj, min) >= (5, 8) ->
  Lwt.return_unit
  | Some maj, Some min ->
- Lwt.fail_with
- (Fmt.str
- "You need at least linux 5.8 to use the btrfs backend, \
- but current kernel version is '%d.%d'"
- maj min)
+ Fmt.failwith "You need at least linux 5.8 to use the btrfs backend, \
+ but current kernel version is '%d.%d'" maj min
  | _, _ ->
  Fmt.failwith "Could not parse kernel version %S" kver
  end

lib/os.ml

@@ -105,8 +105,8 @@ let exec ?timeout ?cwd ?stdin ?stdout ?stderr ?(is_success=((=) 0)) ?(cmd="") ar
  let pp f = pp_cmd f (cmd, argv) in
  !lwt_process_exec ?timeout ?cwd ?stdin ?stdout ?stderr ~pp (cmd, Array.of_list argv) >>= function
  | Ok n when is_success n -> Lwt.return_unit
- | Ok n -> Lwt.fail_with (Fmt.str "%t failed with exit status %d" pp n)
- | Error (`Msg m) -> Lwt.fail (Failure m)
+ | Ok n -> Fmt.failwith "%t failed with exit status %d" pp n
+ | Error (`Msg m) -> failwith m
 
 let running_as_root = not (Sys.unix) || Unix.getuid () = 0
 
@@ -205,7 +205,7 @@ let pread_all ?stdin ~pp ?(cmd="") argv =
  >>= fun (stdin, stdout) ->
  child >>= function
  | Ok i -> Lwt.return (i, stdin, stdout)
- | Error (`Msg m) -> Lwt.fail (Failure m)
+ | Error (`Msg m) -> failwith m
 
 let check_dir x =
  match Unix.lstat x with
@@ -300,4 +300,3 @@ let read_lines name process =
  | Some s -> loop ((process s) :: acc)
  | None -> close_in ic; acc in
  loop []
-