treewide: errors variables should be prefixed with Err

Change-Id: Ic277f98ebcd03356500ce7daba199738e701e81c
Reviewed-on: https://review.monogon.dev/c/monogon/+/3025
Tested-by: Jenkins CI
Reviewed-by: Serge Bazanski <serge@monogon.tech>
Vouch-Run-CI: Tim Windelschmidt <tim@monogon.tech>
diff --git a/metropolis/cli/metroctl/cmd_k8scredplugin.go b/metropolis/cli/metroctl/cmd_k8scredplugin.go
index adc3fe8..0d1f318 100644
--- a/metropolis/cli/metroctl/cmd_k8scredplugin.go
+++ b/metropolis/cli/metroctl/cmd_k8scredplugin.go
@@ -28,7 +28,7 @@
 
 func doK8sCredPlugin(cmd *cobra.Command, args []string) {
 	cert, key, err := core.GetOwnerCredentials(flags.configPath)
-	if errors.Is(err, core.NoCredentialsError) {
+	if errors.Is(err, core.ErrNoCredentials) {
 		log.Fatal("No credentials found on your machine")
 	}
 	if err != nil {