Display infinite loops to user

Also exits early if infinite loop

client/commonFramework/update-preview.js

@@ -11,6 +11,7 @@ window.common = (function(global) {
 <script>
   window.$ = parent.$.proxy(parent.$.fn.find, parent.$(document));
   window.loopProtect = parent.loopProtect;
+  window.__err = null;
 </script>
 <link
   rel='stylesheet'

client/iFrameScripts.js

@@ -10,6 +10,12 @@ window.$(document).ready(function() {
   var tests = parent.tests;
   var common = parent.common;
 
+  window.loopProtect.hit = function(line) {
+    window.__err = new Error(
+      'Potential infinite loop at line ' + line
+    );
+  };
+
   common.getJsOutput = function evalJs(code = '') {
     let output;
     try {