Add a default helperMissing to help catch errors and restore old behavior

lib/handlebars/base.js

@@ -42,6 +42,14 @@ Handlebars.registerPartial = function(name, str) {
   this.partials[name] = str;
 };
 
+Handlebars.registerHelper('helperMissing', function(arg) {
+  if(arguments.length === 2) {
+    return undefined;
+  } else {
+    throw new Error("Could not find property '" + arg + "'");
+  }
+});
+
 Handlebars.registerHelper('blockHelperMissing', function(context, fn, inverse) {
   inverse = inverse || function() {};