This is the way the world ends: with a nanosecond of computer error in a Defense Department laboratory and a million casual contacts that form the links in a chain letter of death. And here is the bleak new world of the day after: a world stripped of its institutions and emptied of 99 percent of its people. A world in which a handful of panicky survivors choose sides-or are chosen. A world in which good rides on the frail shoulders of the 108-year-old Mother Abagail-and the worst nightmares of