api: Tweak handling of NBD_CMD_READ reply sanity check

We have been moving towards a trend of reporting EPROTO (something the
server cannot send over the wire) when diagnosing something the server
did wrong, which is better than the ambiguous EIO.  Furthermore, it is
useful to leave a debug trail when we report an error even when the
(buggy) server reported success.

Fixes: edd8f5ce ("api: Tighter checking of structured read replies", v1.13.1)