The easiest way to do this is run a XHR request 
create this file in PHP for your error script
   <?php
// Headers
header('Access-Control-Allow-Origin: *');
header('Content-Type: application/json');
header('Access-Control-Allow-Methods: POST');
header('Access-Control-Allow-Headers: Access-Control-Allow-Headers,Content-Type,Access-Control-Allow-Methods, Authorization, X-Requested-With');
if($_POST)
{
echo json_encode($_POST);
// USE this portion if it is a success 
//Note weather your data passed or failed you have to respond back to the XHR request in the front end with json_encode...
//Your PHP code 
  echo json_encode(
      array('message' => 'Post Created')
 );
}else{
echo json_encode(
        array('message' => 'Post Not Created')
);
}
?>
This is the XHR Ajax send with a special method to extract the form values 
let xtart = document.getElementById('xhrStart');
xtart.addEventListener('click',function(){console.log('Xstarted')
            // First iterate over the form and get all the form Values
            var element = {};
            var data = new FormData(theForm);
            // Display the key/value pairs
            for(var pair of data.entries()) {
                   // console.log(pair[0]+ ', '+ pair[1]);
                   element[ pair[0].toString() ] = pair[1];
            }
            console.log(element);
            // Time to send the control over to PHP to do its magic
            let xhr = new XMLHttpRequest();
            xhr.open('POST', 'post.php');
            xhr.setRequestHeader("Content-Type", "application/json;charset=UTF-8");
            xhr.responseType = 'json';
            xhr.send(JSON.stringify(element));
            //xhr.send();
            // the response is {"message": "Hello, world!"}
            xhr.onload = function() {
              let responseObj = xhr.response;
              alert(responseObj.message); // Hello, world!
 };
 });
and example of a html form to invoke the XHR
    <form id='theForm'>
<div id="rows">
<div class="sitting-days" style="display:flex; justify-content:center; margin-bottom:20px;">
    <div class="select-date" style="margin-right:30px;">
        <h4 style="text-align:center;">Select Date</h4>
        <input type="date" id="house-sitting-date" name="house_sitting_date" value="">
    </div>
    <div class="yes-no">
        <h4 style="text-align:center;">Yes/No</h4>
        <select name="house_sitting_date_yes_no" id="house-yes-no" style="height:24px;">
            <option value="nada" selected>Please choose an option</option>
            <option value="yes">Yes</option>
        </select>
      </div>
   </div>
</div>
<input type='button' value='Submit' id='xhrStart'>
This is the same method as we use in a rest API and should help your purposes with error and script handling