Added class sonata-ba-field-error for activate tab

Sonata js activate tab with error by "sonata-ba-field-error" css class.
See this - https://github.com/sonata-project/SonataAdminBundle/blob/master/Resources/public/Admin.js#L361
Merge pull request #126 from spolischook/patch-1

Added class sonata-ba-field-error for activate tab