Modify error return with get-if-exists missing condition
Current Behavior
When get-if-exists is used and the request resource doesn't exists, the String "The requested resource doesn't exist"" is returned
New Behavior
Return an object that more closed mimics an actual 404. The goal here being to hit the 404 case in this blacklight-core error handler: https://gitlab.entropy.cc/blacklight/blacklight-render/blob/master/lib/blacklight-express.js#L369