Add a special module for js exception

Author: bobzhang

Diff for: jscomp/core/js_runtime_modules.ml

@@ -50,4 +50,4 @@ let block = "Block"
 let js_primitive = "Js_primitive"
 let module_ = "Caml_module"
 let missing_polyfill = "Caml_missing_polyfill"
-let exn = "Js_exn"
+let caml_js_exceptions = "Caml_js_exceptions"

Diff for: jscomp/core/lam_compile_primitive.ml

@@ -52,7 +52,7 @@ let translate  loc
     -> 
     Js_of_lam_exception.make (E.str s)
   | Pwrap_exn -> 
-    E.runtime_call Js_runtime_modules.exn "internalToOCamlException" args 
+    E.runtime_call Js_runtime_modules.caml_js_exceptions "internalToOCamlException" args 
   | Praw_js_function(arg,block) -> 
     E.raw_js_function arg block
   | Praw_js_code_exp s ->