treewide: add missing error handling

Change-Id: I55ccf3ff490b58f6af93e665c668428acddc8d65
Reviewed-on: https://review.monogon.dev/c/monogon/+/3019
Vouch-Run-CI: Tim Windelschmidt <tim@monogon.tech>
Tested-by: Jenkins CI
Reviewed-by: Serge Bazanski <serge@monogon.tech>
diff --git a/metropolis/node/kubernetes/pki/kubernetes.go b/metropolis/node/kubernetes/pki/kubernetes.go
index a30aeda..66731ae 100644
--- a/metropolis/node/kubernetes/pki/kubernetes.go
+++ b/metropolis/node/kubernetes/pki/kubernetes.go
@@ -337,7 +337,7 @@
 	// Save to etcd.
 	_, err = k.KV.Put(ctx, path, string(key))
 	if err != nil {
-		err = fmt.Errorf("failed to write newly generated key: %w", err)
+		return nil, fmt.Errorf("failed to write newly generated key: %w", err)
 	}
 	return key, nil
 }