High Sierra: "An error has occurred" and app won't finish downloading
I helped someone about three weeks ago with the same issue. What worked for them was resetting the System Management Controller and then starting the download again, but this time with their energy saver settings temporarily changed so that their MBP did not go to sleep at all. 

Resetting the SMC

To reset the SMC on your model MBP, follow these steps:


Shut down your MBP
Unplug the power cable from your Mac
For 10 seconds, press and hold at the same time the shiftcontroloption keys (on the left side of the built-in keyboard) and the power button
After 10 seconds let go of all keys and the power button 
Plug in the power cable
Turn your MBP back on with the power button.


Temporarily change your Energy Saver settings

Now ensure your Energy Saver settings (Apple > System Preferences > Energy Saver) mimic the following:



You can change your settings back again after the download works. If it doesn't work, then you'll know it wasn't some sort of power management issue at your end interrupting the download.

Of course, it goes without saying that you should keep your MBP connected to an AC power source (i.e. the Power Adapter) while trying this.