1015 error: There are 4 possible causes of this error:
The activation server truly is unavailable. Rare, usually fixed within minutes.
Your antivirus or firewall or router's firewall is blocking access to
gs.apple.com on ports 80 and 443.
Your iPhone is jailbroken or otherwise hacked.
The computer you connected to during the restore/update was
used to hack/jailbreak some other iOS device, and its
network database was corrupted during the process.
(Thanks to Lawrence Finch for the wording in
https://discussions,apple.com/thread/4793244?start=90 )