Track successfully imported in output but log says there's an error

Steps to reproduce

The real repro steps would be to send you the file. In the meantime, here what I think should reproduce the same error:

1. Take an OGG track
2. Remove the date tag
3. Import using in-place feature

   $ docker exec -it funkwhale manage import_files $LIBRARY_ID "/music/**/*.ogg" --recursive --noinput --in-place
   2020-03-07 15:16:10,194 funkwhale_api.config INFO     Running with no plugins
   Checking imported paths against settings.MUSIC_DIRECTORY_PATH
   Import summary:
   - 1623 files found matching this pattern: ['/music/**/*.ogg']
   - 1622 files already found in database
   - 1 new files
   Selected options: in place
   For details, please refer to import reference 'cli-2020-03-07T15:16:13.568326+00:00' or URL https://music.example.com/content/libraries/xxxx-library-guid-xxxx/upload?import=cli-2020-03-07T15%3A16%3A13.568326%2B00%3A00
   1/1 Importing /music/Pomplamoose/Hey It's Pomplamoose/10 Goodbye Song.ogg...
   Successfully imported 1 tracks
   For details, please refer to import reference 'cli-2020-03-07T15:16:13.568326+00:00' or URL https://music.example.com/content/libraries/xxxx-library-guid-xxxx/upload?import=cli-2020-03-07T15%3A16%3A13.568326%2B00%3A00

4. See the output saying: "Successfully imported 1 tracks"
5. [bonus] Re-import using the same command line
6. [bonus] See the same output again

What happens?

The stdout says: "Successfully imported 1 tracks", but when you click on the link in the output you'll see the track is flagged "Errored".

Here's the error:

[bonus 1] The sentence "album / release_date: This field may not be blank." made me believe either "album" or "release_date" is wrong, until I understood it represents a hierarchical link. Maybe use > instead of /? (I think it's confusing because meta tags don't have hierarchy). [bonus 2] The link to Read our documentation for this error is broken.

What is expected?

If something went wrong, the output should be aligned with the log.

Context

Funkwhale version(s) affected: 0.20.1

Instance configuration: docker

Debug information:

{
    "source": "file:///music/Pomplamoose/Hey It's Pomplamoose/10 Goodbye Song.ogg",
    "detail": {
        "album": {
            "release_date": [
                "This field may not be blank."
            ]
        }
    },
    "error_code": "invalid_metadata",
    "file_metadata": {
        "date": "",
        "album": "Hey It's Pomplamoose",
        "genre": "Indie",
        "title": "Goodbye Song",
        "artist": "Pomplamoose",
        "position": "10",
        "copyright": "",
        "disc_number": "",
        "album_artist": ""
    }
}