## Overview
When accessing a photo that doesn't exist, either individually (`/a/{photo}`, `/me/photos/{photo}`) or on an album (`/{user}/{album}/{photo}`), we should show a "not found" message. Users should be able to easily navigate back to the main gallery (user photos or an album).
Right now, when logged in, the user simply lands on the parent gallery (user's photos or an album). When logged out, the user gets a raw JSON string.