Upgrader: retry download of mirrors.json on failure
It can happen that our Upgrader fails to download
mirrors.json from our website, and then the user sees:
Error while choosing a download server <b>Could not choose a download server.</b> This should not happen. Please report a bug.
@anonym suggested on #15875 (comment 162439) that we retry this HTTP request with a different Tor circuit, would solve this problem. In other cases (e.g. our web server is rebooting), it probably won't.