invalid item page

[brokkoli71]

User stories

when an item got deleted and the former page will be accessed (e.g. via QR Code), there should be message other than internal error

Acceptance Criteria

• [x] will be shown on deleted items in #348
• [x] says something like "this item might have been removed or never existed"

[antonneubauer]

is done (imo) as of #348 @brokkoli71 please try it out (e.g. by https://bookkeeper-blue-dev.herokuapp.com/items/?locale=en) or scanning an QR-Code of an deleted item