Not a single major summer movie will fail at the box office because it’s bad. Instead, disappointing grosses for certain movies will be blamed on one or more of the following factors: Warm weather. Cold weather. Rainy weather. The recession. Bad marketing. The war. Film festival fever. Unfairly high expectations. Theaters that are filled with just too darn many good movies at once. An unexpected ”backlash.” Tom Cruise (because that just never gets old). Intense national interest in TV’s riveting gavel-to-gavel coverage of the Republican National Convention. Movie piracy. Actual pirates. Fear of avian flu. And Hillary. In cases in which a movie’s performance is so exceptionally poor that it cannot be excused by anything on the above list, we will be sternly reminded that everyone knows the real money these days comes from DVDs. The movie will then fail on DVD. But quietly.