use I18n errors.format key to generate field error message

Rails uses the translation key "errors.format" to generate the error message on fields, instead of just concatenating the field name and the error. This patch uses ActiveModel [`full_message`](https://github.com/rails/rails/blob/127411fdf3a3470e8830abf0c7876db67c0c344a/activemodel/lib/active_model/errors.rb#L286) to do the same.