Improve form errors popovers and colors

I removed the qtip specific code as popovers work on select elements.

But I had to add a new specific code to correctly handle popovers on ``sonata_type_model_list`` fields. I had to replace the "focus" trigger with a "hover" one, else the popover won't show (tell me if you prefer to keep focus on other inputs).

I also added a red border to ``sonata_type_model_list`` fields if there is an error.

![error](https://f.cloud.github.com/assets/663607/911319/ff91e432-fde8-11e2-831c-3c2d0b5c7460.png)