Add error details to errors hash

[AlexVPopov]

In Rails 5 ActiveModel::Errors contains an additional attribute details. For more info, see:

http://api.rubyonrails.org/classes/ActiveModel/Errors.html#method-i-add

http://blog.bigbinary.com/2016/05/03/rails-5-adds-a-way-to-get-information-about-types-of-failed-validations.html

I think this info should be added to her and all the other places, where errors are added. Will such a PR be accepted?