nbd/client: More consistent error messages

nbd/client: More consistent error messages

Consolidate on using decimal (not hex), on outputting the
option reply name (not just value), and a consistent comma between
clauses, when the client reports protocol discrepancies from the
server.  While it won't affect normal operation, it makes
debugging additions easier.

Signed-off-by: Eric Blake <[email protected]>
Reviewed-by: Richard W.M. Jones <[email protected]>
Reviewed-by: Vladimir Sementsov-Ogievskiy <[email protected]>
Message-Id: <20181215135324[email protected]>