NetlinkPayload::Ack/Error are type-unsafe

[ghananigans]

Per 1 and 2, an "ACK" message is a variant of Error (NLMSG_ERROR)
message with a zero value error code and a "NACK" message is a variant
with a non-zero error code.

At TOT, the NetlinkPayload::Ack is meant to be the "ACK" message and
the NetlinkPayload::Error is meant to be the NACK message. The
NetlinkPayload::Ack an NetlinkPayload::Error variants both hold
ErrorMessage. This creates an opportunity for misuse and subtle bugs
where a NetlinkPayload::Ack can hold an ErrorMessage with a non-zero
code or an NetlinkPayload::Error can hold an ErrorMessage with a
zero value code.