GET /eholdings/packages/123 should give an error indicating invalid packageId.
Please Note that packageId is a combination of providerId-packageId.

Mod-kb-ebsco gives an unrelated error response passing the request to RM API:

Validate packageId for different HTTP methods that this endpoint supports and write associated unit tests.

Another example: DELETE /eholdings/packages/abc-abc and DELETE /eholdings/packages/abc each give 404 and 400 with no error message. During validation of url params, ensure that error messaging is consistent.