Using API to get a data element that doesn't exist responds with 500 instead of 404

### Description of the bug

If I have a valid user logged in, that has created an instance with a valid instanceId, and I ask for an not existing but valid data id on the data/{dataGuid} GET endpoint, the server responds with 500 internal server error.

Expected behavior: The server should respond with a 404 not found, as I'm asking for an element that doesn't exist. Potentially a 403 bad request. This works properly if the request is not a properly formatted guid, but with proper guid formatting the server assumes it made a mistake.

### Steps To Reproduce

1. Authenticate as a test user with access to vpfo : `user_id: 160616; party_id: 51882621`
2. Create instance: `POST https://brg.apps.tt02.altinn.no/brg/aarsregnskap-vpfo-202405/instances?instanceOwnerPartyId=51882621`
3. Get data element with a valid Guid that does not exist: `GET https://brg.apps.tt02.altinn.no/brg/aarsregnskap-vpfo-202405/instances/51882621/{{schema_instance_id}}/data/82a351d9-807f-4949-9d6b-3cc7eea38a17`
4. The server will respond with 500

### Additional Information
The bug is not exclusive to VPFO, and should be reproduceable with any app / data element

Start by reproducing the GET /instances/{instanceOwnerPartyId}/{instanceId}/data/{dataGuid} request with a valid, nonexistent GUID, as described in the issue. Trace the resulting 500 response to the API error handling and verify that a missing data element returns 404 without changing the behavior for malformed GUIDs.

Written by the indexing model from the issue text.