nbd: Detect servers that send unexpected error values

Add some debugging to flag servers that are not compliant to
the NBD protocol.  This would have flagged the server bug
fixed in commit c0301fcc.

Signed-off-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Alex Bligh <alex@alex.org.uk>

Message-Id: <1463006384-7734-11-git-send-email-eblake@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>