fix: add ErrUserNotFound and ErrPolicyNotFound to groupErrorToHTTPStatus

Map these sentinel errors to 404 so AddGroupMember and
AttachGroupPolicy return proper HTTP status codes.

weed/admin/handlers/group_handlers.go

@@ -27,6 +27,12 @@ func groupErrorToHTTPStatus(err error) int {
 	if errors.Is(err, credential.ErrPolicyNotAttached) {
 		return http.StatusBadRequest
 	}
+	if errors.Is(err, credential.ErrUserNotFound) {
+		return http.StatusNotFound
+	}
+	if errors.Is(err, credential.ErrPolicyNotFound) {
+		return http.StatusNotFound
+	}
 	return http.StatusInternalServerError
 }