Return only 404 on ActivityPub coll post request

Ref T493

handle.go

@@ -721,6 +721,10 @@ func (h *Handler) handleHTTPError(w http.ResponseWriter, r *http.Request, err er
 			return
 		} else if err.Status == http.StatusNotFound {
 			w.WriteHeader(err.Status)
+			if strings.Contains(r.Header.Get("Accept"), "application/activity+json") {
+				// This is a fediverse request; simply return the header
+				return
+			}
 			h.errors.NotFound.ExecuteTemplate(w, "base", pageForReq(h.app.App(), r))
 			return
 		} else if err.Status == http.StatusInternalServerError {

posts.go

@@ -1352,7 +1352,7 @@ Are you sure it was ever here?`,
 		fmt.Fprint(w, p.Content)
 	} else if strings.Contains(r.Header.Get("Accept"), "application/activity+json") {
 		if !postFound {
-			w.WriteHeader(http.StatusNotFound)
+			return ErrCollectionPageNotFound
 		}
 		p.extractData()
 		ap := p.ActivityObject()