Send a response when changing status of removed entries in Fever API

When a client, like Reeder, tries to mark an item which is removed on Miniflux, the server doesn't response OK. This may terminate the sync process, the PR fixes it.

fever/handler.go

@@ -398,6 +398,8 @@ func (h *handler) handleWriteItems(w http.ResponseWriter, r *http.Request) {
 	}
 
 	if entry == nil {
+		logger.Debug("[Fever] Marking entry #%d but not found, ignored", entryID)
+		json.OK(w, r, newBaseResponse())
 		return
 	}