$error_messages parameter for UploadManager class for localization.

Might be convenient adding a third parameter for the UploadManager constructor for using localized strings for the error messages in UploadManager.php:

Line 37:
function __construct($options = null, $initialize = true, $error_messages = null) {...}
Line 112:
if ($error_messages) {
$this->error_messages = array_merge($this->error_messages, $error_messages);
}