Point every write action's error_handler at AshGettext.ErrorTranslation.ErrorHandler.

Doing this at compile time is what makes translation a property of the resource rather than a thing each action has to remember. Adding an action to a resource that uses AshGettext.Resource cannot forget to translate.

Two deliberate limits:

• Write actions only. Ash 3.29 exposes error_handler on create, update and destroy — the changeset-based actions. Read and generic actions have no equivalent hook, so their errors are not translated here.
• An action that already declares an error_handler is left alone. The app asked for that handler explicitly; silently replacing it would be a worse surprise than an untranslated message.