However, anyone who has used HTTP error codes in REST knows to avoid that – Receiving a 404-Not-Found causes hours of debugging and blind poking compared to receiving a 200-OK-{“error”:”Entity not found”}…
HTTP is just a transport for your RPC. It's an implementation detail. At the HTTP layer, the transport was successful, so a 200 is appropriate.
A 404 would indicate 'not found' at the transport layer e.g. a bad proxy configuration or you didn't hit the right server at all. You definitely wouldn't confuse it with '${my_widget} not found'.
