OCaml, LWT, Rresult - Catching exception before reaching Lwt_main.run

dune

(executable
    (name lwt_demo1)
    (public_name lwt_demo1.exe)
    (libraries rresult lwt lwt.unix)
    (preprocess (pps lwt_ppx))
    (flags (:standard -warn-error -22))
)

execution-error.log

Fatal error: exception Unix.Unix_error(Unix.ENOENT, "open", "a_non_existent_filename")
Raised by primitive operation at file "src/unix/lwt_unix.cppo.ml", line 235, characters 14-31
Re-raised at file "lwt_demo1.ml", line 28, characters 4-233
Re-raised at file "src/core/lwt.ml", line 3027, characters 20-29
Called from file "src/unix/lwt_main.ml", line 27, characters 10-20
Called from file "src/unix/lwt_main.ml", line 114, characters 8-13
Re-raised at file "src/unix/lwt_main.ml", line 120, characters 4-13
Called from file "lwt_demo1.ml", line 38, characters 9-33

lwt-demo1.ml

(*
 I want to catch an exception before it reaches Lwt_main.run. 
 I know I can catch the exception at Lwt_main. Just want to avoid it somehow.
*)

open Rresult

let printLine line =
  let newline = "-> " ^ line in
  let () = print_endline newline in
  newline
;;

let resultOpenFile =
  try%lwt
    Lwt.return (R.ok (Lwt_io.open_file ~mode:Input "a_non_existent_filename"))
  with
  | Unix.Unix_error (uerr, ucommand, dir) ->
    (match uerr with
    | ENOENT -> Lwt.return (R.error ("NEVER CALLED - ENOENT"))
    | _ -> Lwt.return (R.error "NEVER CALLED"))
  | _ -> Lwt.return (R.error "NEVER CALLED")
;;

let mainPromise =
  match%lwt resultOpenFile with
  | Ok wrappedInputChannel ->
    (* If Ok, Let's print result *)
    let%lwt inputChannel = wrappedInputChannel in
    let strLwtStream = Lwt_io.read_lines inputChannel in
    let%lwt resultList = Lwt_stream.to_list strLwtStream in
    let _list = List.map printLine resultList in
    Lwt.return ()
  | Error _ ->
    let () = print_endline "RRESULT Some error happened" in
    Lwt.return ()
;;

let () = Lwt_main.run mainPromise