templates: Fix stack traces throwing handlebars errors.

This fixes the fact that we weren't using `throw` correctly, and also
the "handlerbars" typo.

Fixes #2143.

static/js/templates.js

@@ -4,13 +4,13 @@ var exports = {};
 
 exports.render = function (name, arg) {
     if (Handlebars.templates === undefined) {
-        throw "Cannot find compiled templates!";
+        throw new Error("Cannot find compiled templates!");
     }
     if (Handlebars.templates[name] === undefined) {
-        throw "Cannot find a template with this name: " + name
+        throw new Error("Cannot find a template with this name: " + name
               + ". If you are developing a new feature, this likely"
               + "means you need to add the file static/templates/"
-              + name + ".handlerbars";
+              + name + ".handlebars");
     }
 
     // The templates should be compiled into compiled.js.  In