Add option to create temporary files for partial downloads

[dreamflasher]

Currently, if the process is killed it's impossible to find out if a file was completely downloaded. Thus it would be great to add an option that files are created with a prefix or suffix and renamed to the actual filename only if they are completed.

[Cadair]

If the download is interrupted (but not completely killed) the partially downloaded files should be removed.

I agree that renaming at the end of the download is a good idea, shouldn't be that hard to add.

[dreamflasher]

I also had situations where an error occured, but the partially downloaded file wasn't deleted. My current impression is that the results object is not containing all errors, but I need to do proper debugging (it's a bit challening, because it occured when I downloaded ~1000 files, so there's a bunch of randomness involved).

What do you think, should renaming at the end of the download be the default? (Instead of the callback handler? With the callback handler I was inspired by https://docs.python.org/3/library/asyncio-task.html#asyncio.Task.add_done_callback – but maybe it adds too much complexity here)

[Cadair]

should renaming at the end of the download be the default

yes, I think so.

Instead of the callback handler?

I think from a user perspective they are separate, but we could use the callback handler internally if that makes sense.

[dreamflasher]

Great. Yes, I think the callback handler would be a great usecase for implementing this. Lets finish that PR and then I'll add the temp-file callback handler. Or do you prefer that I add the temp-file callback handler already in the other PR?

[Cadair]

Keeping them separate is a good idea.