Fix form errors

@phiamo, give this a good look over before merging it. I _think_ it's pretty much all set but this could break a lot of stuff if wrong. 

Would fix:
- https://github.com/phiamo/MopaBootstrapBundle/issues/884
- https://github.com/phiamo/MopaBootstrapBundle/issues/877
- https://github.com/phiamo/MopaBootstrapBundle/issues/754
- https://github.com/phiamo/MopaBootstrapBundle/issues/755

Basically moved form_errors out of the form_widget blocks. It will only render errors when you use `form_row` now. There is technically some BC because people who used `form_widget` and not `form_errors` at the same time will not be shown errors anymore, and you wouldn't have really ever done that anyway as it would have printed the errors twice. The errors page on the sandbox looks pretty good.