[plugin_installer] Display error message if fetching plugin list fails
Some time ago, perhaps with the beginning of the 1.3.x versions I lost the ability to update plugins via the list of Available plugins. This list is always empty now. I tried to install recent version of Gajim on two PCs - Win7 and Win10. I tried to remove all previous application data and I also disabled third-party firewall and dnscrypt. Nothing helped. But I can use web browser to access https://ftp.gajim.org/plugins_1.3_zip/ to download and install plugins.