libcni: Fix error handling in GCNetworkList

Return actual error from GetCachedAttachments instead of nil to properly
propagate errors and avoid silent failures when getting cached attachments
fails during garbage collection.

Author: qinqon

libcni/api.go

@@ -776,7 +776,7 @@ func (c *CNIConfig) GCNetworkList(ctx context.Context, list *NetworkConfigList,
 	// First, get the list of cached attachments
 	cachedAttachments, err := c.GetCachedAttachments("")
 	if err != nil {
-		return nil
+		return err
 	}
 
 	var validAttachments map[types.GCAttachment]interface{}