[js] Store the exception message in $$message

Storing it in $$message doesn't cause confusion with the message method.

Author: pmurias

src/vm/js/nqp-runtime/core.js

@@ -506,11 +506,11 @@ op.getpid = function() {
 };
 
 op.getmessage = function(exception) {
-  return exception.message;
+  return exception.$$message;
 };
 
 op.setmessage = function(exception, message) {
-  return (exception.message = message);
+  return (exception.$$message = message);
 };
 
 op.getpayload = function(exception) {

src/vm/js/nqp-runtime/nqp-exception.js

@@ -1,10 +1,11 @@
 class NQPException extends Error {
   constructor(message) {
     super(message);
+    this.$$message = message;
   }
 
   Str(ctx, _NAMED, self) {
-    return this.message;
+    return this.$$message;
   }
 
   $$toBool(ctx) {

src/vm/js/nqp-runtime/reprs.js

@@ -1583,7 +1583,7 @@ class VMException extends REPR {
 
     STable.addInternalMethods(class {
       $$getStr() {
-        return this.message;
+        return this.$$message;
       }
     });
   }