Refactor CLI exit code and error handling

cmd/poweroff.go

@@ -33,8 +33,8 @@ func PoweroffCmd(args CmdArgs) (int, error) {
 
 	r, err := c.SetState(selector, state)
 	if err != nil {
-		return 1, err
+		return ExitError, err
 	}
 	PrintResults(r.Results)
-	return 0, nil
+	return ExitSuccess, nil
 }