Lwt_unix.file_exists can raise Unix.ENOTDIR

[samoht]

Test case:

$ touch foo
$ ocaml
# require "lwt.unix";;
# Lwt_main.run (Lwt_unix.file_exists "foo/bar");;
Exception: Unix.Unix_error (Unix.ENOTDIR, "stat", "_log/toto").
# Sys.file_exists "foo/bar";;
- : bool = false

[mfp]

I went ahead and committed this even though it breaks "bug for bug" compatibility because existent code able to cope with spurious exceptions raised by file_exists must handle the case where it returns false anyway, and it seems very unlikely anybody would rely on these exceptions being raised (and handle them differently to the false case).