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.

error-formatter コンポーネントと #689 の作業から始め、API エラーの種類が現在どのようにメッセージにマッピングされているかを確認します。API エラー一覧の扱い（リダイレクト、ユーザーにわかりやすいローカルメッセージ、または別の動作）について合意し、それに応じてコンポーネントを改善できれば完了です。

索引モデルが issue の本文から書いたものです。