Reproduction steps:
1. Open the attached project.
2. Build and run it on an iOS device.
3. Minimize the application before download finishes.
4. Wait for a minute.
5. Maximize the application.

Actual behavior:
"done = True"
"error = Unknown Error"

Expected behavior:
"done = True"
"error="

Reproduced with:
2017.2.3p3, 2018.1.9f1, 2018.2.5f1, 2018.3.0a9
iPhone 6 iOS 10.3.3, iPod Touch 6 iOS 10.2.1

Not reproducible with:
Android