nbd/client: Trace all server option error messages

nbd/client: Trace all server option error messages

Not all servers send free-form text alongside option error replies, but
for servers that do (such as qemu), we pass the server's message as a
hint alongside our own error reporting.  However, it would also be
useful to trace such server messages, since we can't guarantee how the
hint may be consumed.

Signed-off-by: Eric Blake <[email protected]>
Message-Id: <20181218225714[email protected]>
Reviewed-by: Vladimir Sementsov-Ogievskiy <[email protected]>