Fix custom artwork being lost if the item is moved (for users with no account)

[GianniCarlo]

Bugfix

• After updating the artwork for an item (with no account), if it's moved into a folder (or to a different level), the custom artwork is lost, as the cache key is for the previous relative path

Things to be aware of / Things to focus on

• This also includes a fix for an alert being shown on app launch when trying to load the last book and failing
• It also provides more error details in the alert