added missing action and views

I like how Yiinitializr is minimalistic, so this might be intentional, but because it's an important part of application and without it you get errors (when you should actually get a different error), so I think it might be a good idea to include this.

Now then for example entering a non existing page the front-end throws an exception with a message `The system is unable to find the requested action "error".`.
If entering a non existing page on the back-end it throws an exception with a message `SiteController cannot find the requested view "error".` and of course after adding the action it will give an error about missing view.