Improve check for existing template

There is a long story. You hear about problem with smarty "debugging". It's because in depth yii code renderPartial and render always set $return to "true". And always we have fetch() (not display()). And then always a debug window doesn't render.

If we call renderFile() — ok, we have debug window worked well, and smarty understand path like a "blog/index.tpl", but old code don't.

There is templateExists() — core Smarty function, included in both versions.