Allow re-downloading episodes not found on system

### Expected behavior
If downloaded episodes are lost from the device (for e.g. on SD format), there should be an option to re-download all of them without having to remove them individually from the Pocket Casts downloads list.

### Actual behavior
If downloaded episodes are lost from the device (for e.g. on SD format), they still show up in the downloads screen. The only way to re-download the files is to remove them from the Pocket Casts downloads list and then download them again.

### Steps to reproduce the behavior

- Open the app.
- Go to the `Discover` tab -> tap a podcast -> tap an episode from the selected podcast.
- Download the episode.
- Delete the downloaded episode from the device storage location: `/sdcard/Android/data/au.com.shiftyjelly.pocketcasts.debug/files/PocketCasts/podcasts`
- Go to `Profile` tab -> `Downloads`.
- Notice that the downloaded episode still shows in the downloaded list.
- Try to play the episode.
- Notice that an error is displayed "Episode file not found, please try downloading it again."

The file can now be downloaded only after removing it from the downloads list. It can be troublesome to download all episodes if it's a big downloaded list.

Ticket: 5677123-zd-woothemes

Reproduce the missing-file case by deleting an episode under /sdcard/Android/data/au.com.shiftyjelly.pocketcasts.debug/files/PocketCasts/podcasts, then inspect the Profile → Downloads flow and the displayed "Episode file not found" error. Done means lost downloaded episodes can be re-downloaded without removing them individually from the downloads list.

Written by the indexing model from the issue text.