Show 404 page when requested image id doesn't exist

[obulat]

Fixes #46

The fetch method in photos/_id.vue dispatches two actions concurrently: to fetch the current image, and the related images. If the image fetch action in the store throws an error, it is re-thrown and handled within the fetch method. The fetch method was rewritten to use the nuxt context variables instead of this so that we could use the standard error method:

catch (err) {
      error({
        statusCode: 404,
        message: app.i18n.t('error.image-not-found', { id: route.params.id }),
      })
    }
```

[zackkrida]

Very nice improvements all around, including the new error namespace in the translation strings. Thank you!