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_takeownership.go b/metropolis/cli/metroctl/cmd_takeownership.go
index adad3cf..5161dc9 100644
--- a/metropolis/cli/metroctl/cmd_takeownership.go
+++ b/metropolis/cli/metroctl/cmd_takeownership.go
@@ -47,7 +47,7 @@
 	// Retrieve the cluster owner's private key, and use it to construct
 	// ephemeral credentials. Then, dial the cluster.
 	opk, err := core.GetOwnerKey(flags.configPath)
-	if errors.Is(err, core.NoCredentialsError) {
+	if errors.Is(err, core.ErrNoCredentials) {
 		log.Fatalf("Owner key does not exist. takeownership needs to be executed on the same system that has previously installed the cluster using metroctl install.")
 	}
 	if err != nil {