“Horizon An American Saga Chapter 1″, the sumptuous Warner Brothers production hitting theaters this week, is the type of film that is extremely easy to admire but very difficult to recommend. The film marks Kevin Costner’s return to the director’s chair in a western and his efforts are obviously loaded with ambition. The problem is that even with three hours, an epic canvas, high production values and an excellent ensemble cast, the film fails to fully justify its existence.