Q: system download failure

Hi janrfarmer,

Thanks for using Apple Support Communities for assistance with your OS X download issue. Upgrading your Mac to the latest OS X is a always a great idea to keep it running smoothly. If you are having issues with getting it downloaded form the App store, there are a few things that can be done to resolve this. The first is try using a wired connection if using Wi-Fi. Connecting your Mac to the internet via an ethernet cable will really speed up download times and ensures a stable connection during the download process. 

Next, you can boot your Mac up into safe mode. When you start up in safe mode, your Mac prevents some software, such as startup items, from loading, and it performs a check of your startup disk. Your Mac may take longer to start up because of the check. It is also a good method to use if your Mac is having issues starting up, but in your case, we will just use safe mode to get your OS X downloaded.

Try safe mode if your Mac doesn‘t finish starting up

If after booting up into safe mode and the issue is persistent, try setting up a test user account and downloading it from the App Store in this account. How to test an issue in another user account on your Mac

The procedures in the links above should have your Mac upgraded to El Capitan in no time.

Have a great day.