When converting an mjsunit exception to a string, include the stack

Makes debugging failures a bit easier

BUG=none
R=ulan@chromium.org
LOG=n

Review URL: https://codereview.chromium.org/1398703007

Cr-Commit-Position: refs/heads/master@{#31195}

test/mjsunit/mjsunit.js

@@ -39,7 +39,7 @@ function MjsUnitAssertionError(message) {
 
 
 MjsUnitAssertionError.prototype.toString = function () {
-  return this.message;
+  return this.message + "\n\nStack: " + this.stack;
 };