i18n: translate response code templates

[See how to translate a module](https://github.com/inveniosoftware/invenio-i18n/issues/78)

- [x] Response code templates
- [x] `401.html`
- [x] `403.html`
- [x] `404.html`
- [x] `429.html`
- [x] `500.html`

Read issue #78 for the module-translation process, then inspect the response templates named in this issue: 401.html, 403.html, 404.html, 429.html, and 500.html. Confirm which translations are still missing and whether the checked items reflect completed work; done means the listed response code templates are translated.

Written by the indexing model from the issue text.