Include partial name in 'undefined partial' exception message

lib/handlebars/base.js

@@ -51,7 +51,7 @@ HandlebarsEnvironment.prototype = {
       extend(this.partials, name);
     } else {
       if (typeof partial === 'undefined') {
-        throw new Exception('Attempting to register a partial as undefined');
+        throw new Exception('Attempting to register a partial called `' + name + '` as undefined');
       }
       this.partials[name] = partial;
     }

spec/partials.js

@@ -103,7 +103,7 @@ describe('partials', function() {
     shouldThrow(function() {
       var undef;
       handlebarsEnv.registerPartial('undefined_test', undef);
-    }, Handlebars.Exception, 'Attempting to register a partial as undefined');
+    }, Handlebars.Exception, 'Attempting to register a partial called `undefined_test` as undefined');
   });
 
   it('rendering template partial in vm mode throws an exception', function() {