Fetch an avatar from the cache by its globally unique ID.
It is up to the caller to ensure that file I/O is rate-limited when loading many avatars in parallel, by limiting calls to GLib.LoadableIcon.load.
| id | the globally unique ID for the avatar | 
| Avatar from the cache, or  | 
| GLib.Error | if checking for existence of the cache file failed | 
0.6.0