Provide a variable for writing output with a default of os.Stderr

app.go

@@ -228,11 +228,11 @@ func (a *App) Run(arguments []string) (err error) {
 
 // DEPRECATED: Another entry point to the cli app, takes care of passing arguments and error handling
 func (a *App) RunAndExitOnError() {
-	fmt.Fprintf(os.Stderr,
+	fmt.Fprintf(OutWriter,
 		"DEPRECATED cli.App.RunAndExitOnError.  %s  See %s\n",
 		contactSysadmin, runAndExitOnErrorDeprecationURL)
 	if err := a.Run(os.Args); err != nil {
-		fmt.Fprintln(os.Stderr, err)
+		fmt.Fprintln(OutWriter, err)
 		OsExiter(1)
 	}
 }
@@ -422,7 +422,7 @@ func HandleAction(action interface{}, context *Context) (err error) {
 	vals := reflect.ValueOf(action).Call([]reflect.Value{reflect.ValueOf(context)})
 
 	if len(vals) == 0 {
-		fmt.Fprintf(os.Stderr,
+		fmt.Fprintf(OutWriter,
 			"DEPRECATED Action signature.  Must be `cli.ActionFunc`.  %s  See %s\n",
 			contactSysadmin, appActionDeprecationURL)
 		return nil