Improve error rendering via `error-formatter`

# Problem

Our `error-formatter` component right now is mostly passing through errors received from the API.

That doesn't really make too much sense. API errors are really technical and not that useful to the user. We should compile a list of API errors and decide how to handle them. The options could be

* Redirect to an error route and not display it via `error-formatter` at all
* Display a friendly client-message on the page, via `error-formatter`. This could eventually even be localised.
* Do something else entirely

# References

Work on this sort of started already in #689. I started ignoring the API messages and simply displaying a friendly, locally defined error based on the type of error received from the API, but it needs further refinement.

Commencez par le composant error-formatter et le travail de #689 ; examinez comment les types d’erreur d’API sont actuellement associés aux messages. Le travail sera considéré comme terminé lorsqu’un accord aura été trouvé sur la gestion de la liste des erreurs d’API — redirection, message local convivial ou autre comportement — et que le composant aura été affiné en conséquence.

Rédigé par le modèle d'indexation à partir du texte de l'issue.