[Plugin Installer] Follow up 4ecf3eb (use QgsNetworkManager in plugin installer)

Author: borysiasty

python/pyplugin_installer/qgsplugininstallerinstallingdialog.py

@@ -84,7 +84,7 @@ def requestFinished(self):
     reply = self.sender()
     self.buttonBox.setEnabled(False)
     if reply.error() != QNetworkReply.NoError:
-      self.mResult = self.http.errorString()
+      self.mResult = reply.errorString()
       self.reject()
       del reply
       return