Mac App store download failures and fake complete notification
I spent a long time with customer support on this; several weeks actually.

One of the things you can try is to remove the folder that is listed in the console log when the issue happens. To do this, open console and leave it running. Next attempt the download and when you get the failure message, go back to console and you will see entries which say that system was unable to access a folder. Using finder, open the folder just above the folder that is named with a random sequence of strings.

For eg:
file:///var/folders/nc/qmbww7hd0cl24ylyqnldqxv80000gn/C/com.apple.appstore/409183694/mzps155989934935175015.pkg#Keynote6.pkg

In the above case, delete the folder nc. Make sure the appstore is closed when you do it. Restart the computer after this.

But what really fixed the problem for me was a very simple change.

Click on the wifi symbol on the menu bar and click open network preferences. Click on the location drop down and click edit locations. Remove all the locations in the resulting window and create a new one with a completely new name.

Restart the PC and connect to your network and attempt the download again. This fix is what worked for me. In case you have the same issue, try it out and let me know if it worked.