I was having the same issues today on both my iPhone 5s and iPad Mini 2. Both are on 8.4. I had some pretty hefty updates to download, namely Pages, Numbers and Keynote - almost 1GB worth in total. The update bar (circle) will complete, then it would error out on both devices.
I just did a reboot on both devices (simultaneously holding down the home button + standby button), went back to the app store, and was able to complete the necessary updates without any issues.
My sister was having the same issue with her iPhone 6 today as well, she switched her phone off, then switched it back on, but is still unable to download her updates. Might get her to try the reboot trick to see if that works for her as well.
Weird, but hey, it's working now!