Resume failed update feature

[Wires77]

Description of the problem being solved:

Occasionally, when downloading large updates for PoB, some files would fail. This would cause the entire update process to have to start over. This PR identifies files it had previously tried to download and failed that still exist in the Update folder. If they match what was about to be downloaded, we don't have to download it again. This means eventually retrying enough times might result in a successful download.

Steps taken to verify a working solution:

• Update from a super old version, downloading almost every file
• Observe some files failing and the download failing
• On the next attempt, observe files being skipped that were successfully downloaded

After screenshot:

[Wires77]

Interestingly, some failures aren't happening on download, but on files in general, even if they already exist:

Not sure what's happening in Lua at that time, but that's still going to cause the update to fail even if all the files are downloaded