Improve the reliability of assets download

[HarmfulBreeze]

At the moment, downloading assets isn't handled properly when there is an error, notably timeouts.

They have been quite common lately, so informing the user exactly about what files were missing and how he should fix it could be a good thing to do, instead of just starting overlay and logging to console and overlay_error.log.

Another way to fix these issues might be to extract the files directly from the dragontail zip, or to grab them from the CommunityDragon Raw instead of the CDN which has proven to be unreliable lately.

[HarmfulBreeze]

Progress was made with c66b390. Moving the assets download from the CDragon CDN to Raw has helped a lot.

Another thing that would greatly improve update speed is checking the last-modified response header. If it's set to a date that is more recent than the latest DDragon update, it would update the asset, else it wouldn't download it.

[HarmfulBreeze]

Good progress was made over the last few days. Assets updater is now reliable and fast enough for the issue to be closed.

• Using the Last-Modified response header did improve assets updater performance by a lot.
• Checking for updater success at the end of AssetsUpdater#performDDragonUpdate (and performCDragonUpdate) makes for a simple way to fix download errors on next overlay startup. It's not ideal as you still need to re-download all files (no file list gets written) but as the update is so fast, it is acceptable.
• Removing the champion icons download improved update speed as well.
• Added specific safety checks in case Orianna does not work correctly. It also helped improving updater stability.