do not render input and print errors

fix for issue #884

to render the input and the error-message in one function/block, makes it hard to customize the output. Renderings like 

```
    {{ form_widget(contact_form.name) }}
    {{ form_label(contact_form.name) }}
    {{ form_errors(contact_form.name) }}
```

are not possible. This breaks the parent behavior from symfony.