Show error message if necessary when pdf file is returned to browser

When a user clicks on an icon, a certain emploee_id is transmitted to the server on the basis of which the server makes the calculations and if these calculations are correct, a pdf document should open in a new browser tab (which is currently happening).
But if validation fails, the user must be notified by a pop-up message describing the errors.

Here I can not figure out how to do it. The problem is that on the client responseType = 'blob' - if you change it, for example on json, the pdf document opens with errors.
Throughout the application, data is sent from the server in the form of json:
{"status_code": 200, "status": "success", "text": null, "data": [{...}, ...]}.
But the underlying code I can not fit under this format.

PS: I tried to convert the final output string to the server via base64_encode and pack everything into json of the form:

{ "status": "success", "binary_data": base64_encode($string) } 

and decode on the client through window.atob (string) and substitute it into the constructor new Blob (), but nothing comes out, the document opens with errors

Server code:

  // здесь происходит несколько этапов валидации по различным правилам // в результате чего в $errors формируется строка из ошибок ... if(!$this->form_validation->run('mto_rules')) { $errors .= validation_errors(); } if($errors !== '') { // здесь нужно как-то уведомить пользователя об ошибках // во всем приложении реализовано: // $response['status_code'] = 200; // $response['status'] = 'error'; // $response['text'] = $errors; // .... // тут могут быть вычисления какие-либо // .... // и в самом конце метода, после всех проверок // $this->output->set_status_header($response['status_code']) // ->set_output($response); } // Формируем некий html и переводим в pdf библиотекой TCPDF ... ... $pdf->writeHTML($html, true, false, false, false, ''); // отдаем бинарную строку - ВАЖНО!!! echo $pdf->Output('', 'S'); 

Client code:

  var xhr = new XMLHttpRequest() xhr.open('GET', 'my.php?emploee_id=' + emploee_id, true) xhr.responseType = 'blob' xhr.onload = function(e) { var blob = new Blob([this.response],{type: 'application/pdf'}) var url = window.URL.createObjectURL(blob) window.open(url) } xhr.send()