Change interface of ClaimRootClientAuthToken and ConfirmAuthToken

Add RootClientTokenOwner argument to check if this owner can claim
device.
Add ErrorPtr to return error in privet response.

BUG=25766815

Change-Id: I508c934e23092514e37b1f4790f0f1e693583ae1
Reviewed-on: https://weave-review.googlesource.com/1939
Reviewed-by: Alex Vakulenko <avakulenko@google.com>