Starting about half an hour ago, the App Store is currently experiencing errors where apps do not finish downloading (or updating). Some users are reporting obtuse error messages, like the one above, whereas others are simply left with paused downloads on their iOS and Mac devices.
Availability likely varies by App Store region, but Twitter reports of App Store issues started around 6.30 Pacific Time. The problems are sporadic and do not necessarily arise on every device, but have been seen to occur across operating system versions: it is not limited to iOS 8 and Yosemite (which is still in beta).
Apple is yet to update its system service status page to acknowledge any issues.