Update etcd snapshot error message to be more informative when etcd database is not found (#3568)

pkg/cli/etcdsnapshot/etcd_snapshot.go

@@ -89,7 +89,7 @@ func run(app *cli.Context, cfg *cmds.Server) error {
 		return err
 	}
 	if !initialized {
-		return errors.New("managed etcd database has not been initialized")
+		return fmt.Errorf("etcd database not found in %s", dataDir)
 	}
 
 	cluster := cluster.New(&serverConfig.ControlConfig)