InfluxDB 2: Deleting not exists label or label with not exists resource should produce `404 Not Found` response

It's should works for all types of resource:

- Dashboards
- Notifications
- Checks
- TelegrafConfigs
- NotificationRules
- Buckets
- Variables
- Tasks
- ...

```bash
# Delete label for not exists resource
--> DELETE http://127.0.0.1:9999/api/v2/buckets/ffffffffffffffff/labels/ffffffffffffffff

<-- 204 No Content http://127.0.0.1:9999/api/v2/buckets/ffffffffffffffff/labels/ffffffffffffffff
```

__Expected behavior:__
Produce `404` HTTP response.

__Actual behavior:__
Produce `204 No Content` HTTP response.

__Environment info:__

* InfluxDB version: latest nightly build

cc @AlirieGray

Start by reproducing the DELETE request shown in the issue, then trace the API handlers for labels across the listed resource types. Done means deletion of a missing label or missing resource consistently returns HTTP 404 rather than 204 for all applicable resources.

Written by the indexing model from the issue text.