Unknown config field only yields warning, continues boot

Bug

If I put in a key in my ignition config that is unused should my system continue to boot? In my example I was handcrafting an ignition config (shame on me) based on ignition snippets provided by another user and I placed the filsystems key at the toplevel and not under the storage key. My system booted, but my filesystems weren't set up as I would expect.

The ignition config used:

{
  "ignition": {
    "version": "3.0.0"
  },
  "systemd": {
    "units": [
      {
        "name": "example.service",
        "enabled": true,
        "contents": "[Service]\nType=oneshot\nExecStart=/usr/bin/false\n\n[Install]\nWantedBy=multi-user.target"
      }
    ]
  },
  "filesystems": [
    {
      "device": "/dev/sdc",
      "format": "ext4",
      "label": "docker",
      "path": "/var/lib/docker",
      "wipeFilesystem": true
    },
    {
      "device": "/dev/sdb",
      "format": "ext4",
      "label": "dockerdata",
      "path": "/dockerdata",
      "wipeFilesystem": true
    }
  ],
  "passwd": {
    "users": [
      {
        "name": "core",
        "groups": [
          "sudo"
        ],
        "sshAuthorizedKeys": [
          "ssh-rsa AAAAB..."
        ]
      }
    ]
  }
}

The logs do show a warning:

op(1): [finished] loading QEMU firmware config module
parsing config with SHA512: 9e8c794cc53fd4cab4ecf965cfe232f85047d5a9a77f71662ee23de3e2fb92be2731df44beab20ec7959e767871e9f4c3194eb3b7ad3c985ca1fc365b3f8316e
warning at $.filesystems, line 10 col 3: Unused key filesystems
disks: disks passed
Ignition finished successfully

Which is the same thing that ignition-validate shows:

[core@localhost run]$ sudo cat /sys/firmware/qemu_fw_cfg/by_name/opt/com.coreos/config/raw | ignition-validate - 
warning at $.filesystems, line 10 col 3: Unused key filesystems
[core@localhost run]$ echo $?
0

but it's just flagged as a warning and ignition-validate returns with a successful exit code.

Operating System Version

• fedora/x86_64/coreos/testing at 30.20190905.0

Ignition Version

• ignition-2.0.1-2.gite75cf24.fc30.x86_64

Environment

QEMU

Expected Behavior

Probably this should error? I guess that's what I'm here to discuss.

Actual Behavior

Warning with no erroring out.

Start with the config-validation path used by ignition-validate and the boot parser, reproducing the supplied JSON and warning output. Determine the intended handling of an unused top-level key, then add or update validation coverage so the validator’s exit status and boot behavior match that decision.