Fix lupdate errors

I suspect lupdate isn't smart enough to figure out what tr() is suppose
to do, so just make it a static function in class.
The error was: tr() cannot be called without context

src/base/net/downloadhandler.h

@@ -30,10 +30,11 @@
 #ifndef NET_DOWNLOADHANDLER_H
 #define NET_DOWNLOADHANDLER_H
 
+#include <QNetworkReply>
 #include <QObject>
+
 #include "downloadmanager.h"
 
-class QNetworkReply;
 class QUrl;
 
 namespace Net
@@ -68,6 +69,8 @@ namespace Net
         void assignNetworkReply(QNetworkReply *reply);
         void handleRedirection(QUrl newUrl);
 
+        static QString errorCodeToString(QNetworkReply::NetworkError status);
+
         QNetworkReply *m_reply;
         DownloadManager *m_manager;
         const DownloadRequest m_downloadRequest;