Review and updated tests for delete

[nitrosx]

Summary

@minottic discovered and reported that the delete endpoints are returning 200 if the document is not deleted.
This is hard to test and, to be consistent, we should return a different code. He suggests 404.
This issues has been derived from PR #662

Current Behaviour

if you delete a document, the endpoint return 200 no matter if the document has been deleted or not.

Expected Behaviour

When the delete endpoint is not able to delete the requested document, it should return a code different than 200.
@minottic suggests a 404