For example if diag module is not built then we should
not spam with error message but we will print an error
if only a particular socket is really used.

v2:
 - continue passing @ns into error handler because it should
   be balanced with receieve handler by number of arguments;
   while we don't use it yet in our handlers better to reserve
   it immediately for future use;

 - strictly speaking probing raw diag module has not much sense
   by now -- the kernel doesn't support its autoloading in some
   versions; still for code consistency we should do the same
   things for all modules.
Signed-off-by: Cyrill Gorcunov <gorcunov@gmail.com>
Signed-off-by: Andrei Vagin <avagin@gmail.com>