ios 8.0.1 - download completed but unable to install
Apple revoked iOS 8.0.1 last night after issues with iPhone 6 models were revealed.

The installation is failing as the device can't verify the download is correct as iOS 8.0.1 is no longer available.

To delete the download, go to:
Settings app -> General -> Usage -> Manage Storage 

Find the iOS 8.0.1 option, select it and tap delete.

Finally, go back to the Software Update screen and it will show iOS 8.0 as up to date.