Client should reject file downloads with empty download URLs

[Rytiss]

Describe the bug If a file contains an empty download_url field, the download never gets completed. Instead, the file gets stuck in the download queue with a transient download error on each attempt.

Expected behavior Such downloads should get rejected immediately with a permanent download error.

Additional context I don't think it is possible for a standard work generator to generate such a file record. Nevertheless, custom generators like mine can.

[AenBleidd]

@Rytiss, do you have some test server to verify this? For me the fix is pretty obvious, but I don't know how to verify it without test server with invalid tasks

[Rytiss]

I can test if you send me a Windows binary with the changes.

[AenBleidd]

Try this link: https://drive.google.com/open?id=10LH1z19zNi97YNFVGrKdYEnP5jV5QL5l

[AenBleidd]

I also sent it to you in Slack