In that case you can catch the errors and notify the user , with your own msg
// code snippet
var file:FileReference = new FileReference();/// make this as globla variable
file.addEventListener(Event.SELECT,onFileSelect);
file.addEventListener(Event.CANCEL, onCancelHandler);
var fileFilters:Array = [];
fileFilters.push(new FileFilter("Images", "*.gif;*.jpeg;*.jpg;*.png;*.tif"));
file.browse(fileFilters);
function onFileSelect(event:Event):void
{
file.addEventListener(Event.COMPLETE,onFileLoad);
file.addEventListener(IOErrorEvent.IO_ERROR,onFileLoadError);
file.load();
}
function onFileLoadError(event:IOErrorEvent):void
{
Alert.show("Show Your msg here");
}