3.0 - add validation errors in variables panel

This add the ability to see the validation errors in the Variable panel.

it ref ref #238. 
As a first step it's limited to top level view variables.
I tested it with entities and form with ArrayContext.
It might be only a bad implementation from my part.
here is the case, I have a Contactontroller, no model so I just wan't to make validation and use the ArrayContext.
here's the code https://gist.github.com/antograssiot/dc228591dce4ba856021.
Was it the correct way of doing this to get standard errors displayed in the Form ?