/dashboards/{dashboardID}/cells PUT - cell with inexistant ID returns HTTP 422 with non-standard message

__Steps to reproduce:__
List the minimal actions needed to reproduce the behavior.

Over the API

1. create a dashboard
2. add some cells
3. update the cells with PUT but in the request body use a random ID for at least one of the cells

__Expected behavior:__
Expected an error like 422 which does occur and with the suggested message 'Unprocessable Entity'
https://developer.mozilla.org/en-US/docs/Web/HTTP/Status/422

__Actual behavior:__
The actual message currently returned is 'conflict'. This is out of sync with the map of error codes to error messages used in grace tests.

__Environment info:__

testing against K8S-IDPE. Latest commit.

```
commit f96c81a91d834081e1ee8b436eaf924346934f67 (HEAD -> master, origin/master, origin/HEAD)
Author: influx-acs[bot] <107396960+influx-acs[bot]@users.noreply.github.com>
Date: Thu Dec 8 07:17:45 2022 +0000

```

Start by reproducing the PUT /dashboards/{dashboardID}/cells request with a nonexistent cell ID, then inspect the error-code-to-error-message mapping referenced in the issue and its grace tests. Done means the endpoint still returns HTTP 422 and uses the expected “Unprocessable Entity” message instead of “conflict”.

Written by the indexing model from the issue text.