UE 4.14.1 on Mac.
The error message in the output log is incorrect when the development package is successfully installed on iOS but fails to launch.
The current error message was valid in Xcode 5 and earlier:
PackagingResults:Error: Error Launch failed! Failed to launch on device. Make sure your device has been enabled for development from within the Xcode Devices window.
That’s just a useless red herring now, though, as that functionality no longer exists in the devices window. The actual fix today (Xcode 8.1, iOS 10.1) is to be using a correct mobile provision AND the device needs to be currently unlocked (can’t be on the lock screen).
[I submitted pull request 3024 with a patch to fix the error message here.]