Retry failed filter list downloads
Background
Outdated filters are the cause of the most common problems users are reporting. Currently, there's no mitigation for failed downloads (except the fallback behavior to handle filter lists that no longer exist), which means that users will keep the outdated filters until the next regular download attempt is made - even if the problem was only temporary (e.g. dropped connection, minor server outage). Therefore we should attempt the download again shortly after it failed, in order to minimize the negative impact a minor server/network problem can have on the user experience.
As discussed in https://gitlab.com/eyeo/adblockplus/abpui/adblockplusui/-/issues/484, it's important to consider server load as well as making those download attempts distinct from regular downloads.
What to change
See outlined approach in https://gitlab.com/eyeo/adblockplus/abpui/adblockplusui/-/issues/484.