Handle when file cannot be fetched from Google Cloud Storage

[rtibbles]

When a file cannot be fetched from Google Cloud Storage it raises an AttributeError as we attempt to download_to_file from None. Instead, we should raise a FileNotFoundError.

Fixing this should give more meaningful error messages when this happens during publishing, and also be properly handled by the set_default_file_duration management command.

[sentry]

Sentry issue: STUDIO-F8W

[rtibbles]

Fixed in #3509