Lib: small refactoring around js exception printing (#1743)

Author: hhugo

lib/js_of_ocaml/js.ml

@@ -812,15 +812,16 @@ let parseFloat (s : js_string t) : number_t =
   let s = Unsafe.fun_call Unsafe.global##.parseFloat [| Unsafe.inject s |] in
   if isNaN s then failwith "parseFloat" else s
 
-let _ =
-  Printexc.register_printer (function
-      | Js_error.Exn e -> Some (Js_error.to_string e)
-      | _ -> None)
-
 let _ =
   Printexc.register_printer (fun e ->
-      let e : < .. > t = Obj.magic e in
-      if instanceof e array_constructor then None else Some (to_string e##toString))
+      if instanceof (Obj.magic e : < .. > t) error_constr
+      then
+        let e = Js_error.of_error (Obj.magic e : error t) in
+        Some (Js_error.to_string e)
+      else
+        match e with
+        | Js_error.Exn e -> Some (Js_error.to_string e)
+        | _ -> None)
 
 let export_js (field : js_string t) x =
   Unsafe.set