refactor: Log server response on version check error

Qt `errorString()` only contains the error code and error text, but not the response body. And, in HTTP/2 the error string is gone but the error is still printed like it's there: https://bugreports.qt.io/browse/QTBUG-140126

src/lib/gui/VersionChecker.cpp

@@ -39,6 +39,7 @@ void VersionChecker::replyFinished(QNetworkReply *reply)
   const auto httpStatus = reply->attribute(QNetworkRequest::HttpStatusCodeAttribute).toInt();
   if (reply->error() != QNetworkReply::NoError) {
     qWarning("version check server error: %s", qPrintable(reply->errorString()));
+    qWarning("version check server response: %s", qPrintable(QString(reply->readAll())));
     qWarning("error checking for updates, http status: %d", httpStatus);
     return;
   }