Let Cobra do the usage error logging

Fixes #1472

commands/hugo.go

@@ -67,7 +67,10 @@ var Source, CacheDir, Destination, Theme, BaseURL, CfgFile, LogFile, Editor stri
 func Execute() {
 	HugoCmd.SetGlobalNormalizationFunc(helpers.NormalizeHugoFlags)
 	AddCommands()
-	utils.StopOnErr(HugoCmd.Execute())
+	if err := HugoCmd.Execute(); err != nil {
+		// the err is already logged by Cobra
+		os.Exit(-1)
+	}
 }
 
 //AddCommands adds child commands to the root command HugoCmd.