Improve handling of existing and broken files

[M-rcus]

Some files error out mid-download, but are still partially written to disk. yiff-dl currently only checks if the file exists and not if it's a valid file.

An easy way of making sure the file is valid is by comparing the filesize (bytes) and comparing it with the Content-Length HTTP header. From there we have a few options:

1. Yiff.party sends the Accept-Ranges: bytes header, so we can specify Range header and continue downloading from where we left off.
   • We'd essentially get the current 'length' (size in bytes) of the file and specify Range: 12355- which should give us the rest of the file.
2. Start downloading it from the beginning.

The first option has a few benefits:

• Less load on Yiff.party (we don't want to be assholes)
• Less bandwidth required (helpful for slower connections, especially for 'larger' files).
  • Though with that said, I don't know the max filesize limit for Patreon uploads are. I've seen a lot of creators rely on 3rd party filesharing sites such as Mega and Google Drive. ¯\_(ツ)_/¯

The second option allows us to make sure the file is downloaded correctly. I haven't experienced it myself, but assuming files can actually be corrupted even if "fully" downloaded, then the second option would be best.
I suspect that an issue like that would be very rare and it's just as easy to just manually remove the file and start yiff-dl again.

[M-rcus]

Closing due to the project being archived: https://github.com/M-rcus/yiff-dl#archived