net: libnetlink -- Call error handler instead of unconditional error message

To be able to zap error messages when needed. Without error
handler we will continue printing error. This will placate
diag modules probing since they are not mandatory features.

Signed-off-by: Cyrill Gorcunov <gorcunov@gmail.com>
Signed-off-by: Andrei Vagin <avagin@gmail.com>