Its an error at Apples end nothing wrong with your system. Usually shows up when Apple pushes a lot of updates at once. Wait a while and try again but it may take overnight before it clears up.
If your desperate for the updates and its not an App Store purchase you can usually find the dmg file of the update on the Apple support site. Search around.
Download and install this update manually and this will resolve your problem. You can get this upgrade from here: http://swcdn.apple.com/content/downloads/53/02/041-8081/2jwp4wjrwygtm4lc608qy4h0 n4a9yyq37g/Safari6.0.2Mountain.pkg
It did work for me