Providing invalid input in the X-Ai-InstanceOwnerIdentifier header results in an internal server error

[SandGrainOne]

Description of the bug

The new input parameter (header) X-Ai-InstanceOwnerIdentifier doesn't handle cases where Register returns bad request. It results in an internal server error.

If the register lookup returns 400, then the operation as a whole should return a Bad Request response.

Steps To Reproduce

Tested in AT22: Endpoint: https://platform.at22.altinn.cloud/storage/api/v1/instances?org=ttd

1. Create a valid token for application owner ttd using token generator.
2. Craft a request where the party value in X-Ai-InstanceOwnerIdentifier is invalid . E.g
   1. X-Ai-InstanceOwnerIdentifier=organisation:test
   2. X-Ai-InstanceOwnerIdentifier=person:42
3. Run the request and observe the error message.

[SandGrainOne]

Tested, approved