Fixes crashing when reverting a model linked to non-existent associate

Author: serge1peshcoff

config/locales/history_rollback.en.yml

@@ -12,3 +12,4 @@ en:
       rollback_success: "Rolled back changes to this record."
       rollback_impossible: "Couldn't rollback. Sorry."
       version_not_found: "Version does not exist."
+      rollback_violates_foreign_key: "Couldn't rollback: this model links to another model, which doesn't exists."

config/locales/history_rollback.ru.yml

@@ -12,3 +12,4 @@ ru:
       rollback_success: "Изменения данной версии восстановлены."
       rollback_impossible: "Невозможно восстановить. Извините."
       version_not_found: "Данная версия не существует."
+      rollback_violates_foreign_key: "Невозможно восстановить: эта модель ссылается на другую модель, которой не существует."

lib/rails_admin_history_rollback/config/actions/history_index.rb

@@ -56,6 +56,8 @@ class HistoryIndex < Base
               rescue ActiveRecord::RecordNotFound
                 flash[:error] = t 'admin.history_rollback.version_not_found'
                 redirect_to :action => :history_index
+              rescue ActiveRecord::InvalidForeignKey
+                flash[:error] = t 'admin.history_rollback.rollback_violates_foreign_key'
               end
 
               redirect_to :action => :history_index