media_discoverer.c: Raise an exception on error.

src/control/media_discoverer.c

@@ -90,6 +90,7 @@ libvlc_media_discoverer_new_from_name( libvlc_instance_t * p_inst,
     if( !p_mdis->p_sd )
     {
         free( p_mdis );
+        libvlc_exception_raise( p_e, "Can't find the services_discovery module named '%s'", psz_name );
         return NULL;
     }