Break compatibility to wrap modules

This module does not seem to be used by public libraries on opam.
I don't think this will break a lot of foreign code.

lib/dune

@@ -2,7 +2,6 @@
   (public_name ojs)
   (synopsis "Runtime support for gen_js_api")
   (libraries)
-  (wrapped false)
   (foreign_stubs (language c) (names ojs_runtime_stubs))
   (modes byte)
   (js_of_ocaml (javascript_files ojs_runtime.js)))

lib/ojs.ml

@@ -116,3 +116,26 @@ let is_null x =
 
 let obj_type x =
   string_of_js (call (pure_js_expr "Object.prototype.toString") "call" [|x|])
+
+module Exn = struct
+  type nonrec t = t
+
+  let name x = string_of_js (get x "name")
+  let message x = string_of_js (get x "message")
+  let stack x = option_of_js string_of_js (get x "stack")
+  let to_string x = string_of_js (call x "toString" [||])
+
+  exception Error of t
+
+  let () = Callback.register_exception "jsError" (Error (obj [||]))
+
+  (* The js_of_ocaml runtime expects to have this registered.
+     So it's probably a bad idea to use both this Ojs.Exn module
+     and the js_of_ocaml standard library. *)
+
+  let () =
+    Printexc.register_printer (function
+        | Error x -> Some (to_string x)
+        | _ -> None
+      )
+end

lib/ojs.mli

@@ -126,3 +126,17 @@ val obj_type: t -> string
     "[object Null]"
     "[object Boolean]"
 *)
+
+module Exn : sig
+  (** OCaml view on JS exceptions *)
+
+  type t
+
+  val name: t -> string
+  val message: t -> string
+  val stack: t -> string option
+  val to_string: t -> string
+
+  exception Error of t
+
+end