net: openthread: Fix ignored return values in OT utils

Do not ignore return values of OT functions registering IPv6 addresses.
As this is called from the net_mgmt callback context, which returns
void, there's not much that can be done, other than logging the error.

To simplify the logging code and avoid the need to use extra buffer,
use net_sprint_ipv6_addr() networking utility function within the log
message to convert binary address to string.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>