Forms: throw exception when missing call parent::__construct()

Instead of this confusing notice
![snimek obrazovky 2014-02-14 v14 33 55](https://f.cloud.github.com/assets/557455/2171116/ac9a6070-957c-11e3-8fb2-df5f70bb78af.png)

You get better exception with descriptive message
![snimek obrazovky 2014-02-14 v14 35 31](https://f.cloud.github.com/assets/557455/2171125/f2c1ca7a-957c-11e3-972f-7cef5bc09ff5.png)