A squinty-eyed, cigarillo-chomping gunslinger of nebulous motives played by an actor with the last name Eastwood. An abducted woman. A few bloody shootouts. Some lovely footage of the American frontier, as played by a foreign country. On the surface, “Diablo” would seem to have all of the proper ingredients for a rollicking retro Western, yet its sights are set a bit higher, which inspires both admiration for its moxie and disappointment that its script and direction aren’t up to the challenge.