treewide: cleanup error string formatting

Change-Id: I9012ba58dded916984468219b214200144a439b9
Reviewed-on: https://review.monogon.dev/c/monogon/+/3023
Reviewed-by: Serge Bazanski <serge@monogon.tech>
Vouch-Run-CI: Tim Windelschmidt <tim@monogon.tech>
Tested-by: Jenkins CI
diff --git a/metropolis/test/launch/cluster/cluster.go b/metropolis/test/launch/cluster/cluster.go
index ca9becd..32c351f 100644
--- a/metropolis/test/launch/cluster/cluster.go
+++ b/metropolis/test/launch/cluster/cluster.go
@@ -495,7 +495,7 @@
 		}
 		return n, nil
 	}
-	return nil, fmt.Errorf("no such node.")
+	return nil, fmt.Errorf("no such node")
 }
 
 // Gets a random EUI-48 Ethernet MAC address