Validation error messages on themed forms are no longer removed

In the default branch of this gem, applying bootstrap:themed to scaffold-generated forms will remove the validation error messages functionality (see issue #747). This is a dangerous default behavior because users may not know that this has been removed, nor what they should do in order to regain the functionality. 

I restored the functionality and also applied Bootstrap-compliant styling to the validation error message (see screenshot) in order to improve the default behavior of this gem for those who don't install SimpleForms.

![bootstrap-rails-with-validation-errors](https://cloud.githubusercontent.com/assets/801116/3719061/aee1bfde-163e-11e4-9af8-9fbb8eac6882.PNG)