Error: 0x80073712 when installing KB 2919355

That's a great question, seeming as the only way to gain access to an 8.1 iso was when a person upgraded from 8 to 8.1 and chose the option to save the files to usb or iso.  Since the Microsoft application that was used to upgrade from 8 to 8.1 no longer accepts valid or generic product keys, it now becomes impossible to even create a virtual machine with Hyper-V, install 8, download the Microsoft 8.1 update application and save the installation files.

While that was the easiest way to gain access to them, the other more difficult route is to run 8 from a virtual machine and use the windows store to update.  The problem is, I can't recall where Microsoft downloads the install files too... if memory serves right, it was to the user's temp files in app data - local, however I can't find the bookmark of the page that gave instructions on where to find the downloaded files at.

Seeming as Microsoft created this problem they should be the ones to make the media available to users, and while you expect malware to corrupt system files, you never dream of an OS manufacturer corrupting their own OS's system files via their own update and update software... this level of corruption is literally unheard of.  And since Microsoft refuses to give access on their site, other than subscription based MSDN or TechNet, to an 8.1 ISO, the only way to get the ISO if the user didn't take steps to when they upgraded from 8 is to download the ISO from a software forum and check the sha1 hashes to make sure the ISO hasn't been tampered with.

EDIT:

While I don't normally suggest software forums for a number of reasons, I don't agree with telling anyone they should have to wipe their system, reinstall 8, and go through the tedious hassle of reinstalling every bit of software simply because Microsoft screwed the pooch, took the cake, and ate it too.  I have little patience with Microsoft at the moment due to their idiocy of refusing to give users actual troubleshooting steps that would do something, instead of the cookie cutter response the same microsoft moderator (the one whom started this thread on page 1) gave on 4 or 5 of the 6 forum threads dedicated to the top 6 errors received from the mucked up KB2919355 install.  As far as Microsoft goes, downloading an 8.1 ISO from a software forum shouldn't be a T & C issue since Windows 8.1 is free and does not have it's own line of Activation Keys.  Since users downloading it from a Software site would be doing it to fix a windows installation that's already been activated, I would hope moderators would give a little leeway on this subject... although I also understand if none can be given.

You mean download media from potentially questionable sources.

JW it's a security update.  One that Microsoft has the responsibility of fixing.  Yet they are not talking responsibility here and letting folks struggle and use questionable downloads to fix their systems.

I'm confused with these instructions.  Where does one get legal access to 8.1 media?

"Run the Windows 8.1 installer from within Windows, as if you're going to do an install repair "upgrade"."

Without 8.1 media, folks cannot get fixed.