[Fix] Fixed potential endless loop when getting error opening url.

- when there is navigating error to http(s) page and WebKit throw
  unsupported content error, there is possibility to stuck in
  endless new tab opening loop when user has QupZilla as provider
  for http(s) scheme

src/webview/webpage.cpp

@@ -223,7 +223,13 @@ void WebPage::handleUnsupportedContent(QNetworkReply* reply)
 
     case QNetworkReply::ProtocolUnknownError:
         qDebug() << "WebPage::UnsupportedContent" << url << "ProtocolUnknowError";
+
+        // We are not going to end in endless new tab opening loop in case
+        // user has QupZilla as default provider for http / https urls
+        if (!url.scheme().startsWith("http")) {
             QDesktopServices::openUrl(url);
+        }
+
         reply->deleteLater();
         return;