The app should retry downloading if it can't load a media

[melix99]

Currently we expect that all the media we want to load always exist when tdlib says that they're downloaded, but in reality the files in the cache could vanish for various reasons (e.g. someone clearing the cache while the app is running), so it's better to just redownload them in case we can't find them instead of panicking.

[melix99]

Thanks to the changes introduced here the app should not crash anymore. But we still need to add the code to re-download the media.