Silence usage on errors.

Also remove redundant print of errorrs; cobra already does this.

Fixes #1

cmd/root.go

@@ -161,14 +161,14 @@ var RootCmd = &cobra.Command{
 	Short: "A command line tool for Dropbox users and team admins",
 	Long: `Use dbxcli to quickly interact with your Dropbox, upload/download files,
 manage your team and more. It is easy, scriptable and works on all platforms!`,
+	SilenceUsage:      true,
 	PersistentPreRunE: initDbx,
 }
 
 // Execute adds all child commands to the root command sets flags appropriately.
 // This is called by main.main(). It only needs to happen once to the rootCmd.
 func Execute() {
 	if err := RootCmd.Execute(); err != nil {
-		fmt.Println(err)
 		os.Exit(-1)
 	}
 }