If you directly bought online before the introduction of the MAS, you can try to download the recent Aperture update from the Support page:
P.S: your profile signature states Mac OS X 10.8.1.
The recent Aperture update requires OS X Lion 10.7.5 or OS X 10.8.2 or later
For future reference if you get that that error message while trying to update it is an indication that Apple is having troubles at their end. There is nothing wrong with your system.
This usually shows up when Apple has pushed out a lot of updates at one time and have problems keeping up with demand.
If you wait a while (hours or overnight) the problem will usually resolve itself.
thanks for making it clear.
in the present case, i've been trying to upgrade for the last 14 hours, which is why i decided to post.
what's more, Safari's upadate wouldn't download either, i had to download it from a link provided by Macworld on their blog.
(but Macworld did mention that Aperture's update wouldn't download).