Set error http response code when occurs error in notify action.

Without this change PayU never resends notifications.

Author: pplotka

src/Action/NotifyAction.php

@@ -93,7 +93,7 @@ public function execute($request): void
                     }
                 }
             } catch (OpenPayU_Exception $e) {
-                throw new HttpResponse($e->getMessage());
+                throw new HttpResponse($e->getMessage(), 500);
             }
         }
     }