dune.nix: Include dune version in warning when dune is not supported

as dune 1 and dune 2 have different version ranges here

pkgs/development/tools/ocaml/dune/1.nix

@@ -2,7 +2,7 @@
 
 if !lib.versionAtLeast ocaml.version "4.02"
 || lib.versionAtLeast ocaml.version "4.12"
-then throw "dune is not available for OCaml ${ocaml.version}"
+then throw "dune 1 is not available for OCaml ${ocaml.version}"
 else
 
 stdenv.mkDerivation rec {

pkgs/development/tools/ocaml/dune/2.nix

@@ -1,7 +1,7 @@
 { lib, stdenv, fetchurl, ocaml, findlib }:
 
 if lib.versionOlder ocaml.version "4.08"
-then throw "dune is not available for OCaml ${ocaml.version}"
+then throw "dune 2 is not available for OCaml ${ocaml.version}"
 else
 
 stdenv.mkDerivation rec {