iTunes could not connect to this iPad. An unknown error occurred (0xE8000015)

This error means that iPad is currently disabled and the only option is to restore it.

You cannot update, only restore.

Here are the steps:

1. Disconnect your device if connected to iTunes
2. Make sure you have most recent version of iTunes installed.
3. Get your device in recovery mode
4. Hold power and home button together until device in recovery mode until you see "connect to iTunes" message on screen
5. Plug your device with USB to iTunes
6. iTunes will give you message to update or restore
7. Click restore.

HTH