How to Display Error Message While Downloading Report

Hello Experts!

I have a php page where students enter information to check their results.
See here

I want to display relevant error message on the same page when wrong info is entered. Currently, I'm using error_reporting(0);.
The result is in PDF format.

Please use following to get accurate result:
Class:                       SS 1A
Session:                  2018/2019
Term:                      1st Term
Adminssion No:    GSS/0001
Password:              abc (could be any string)

And use random data to get what I'm taking about.

Bellow is part of the codes:

myForm:

<form id="resultForm" action="terminal_result.php" method="POST">
                        <!--Form with header-->
                        <div class="card wow fadeIn white" data-wow-delay="0.3s">
                            <div class="card-body">
                                <!--Header-->
                                <div class="form-header blue-gradient">
                                    <h3><i class="fa fa-download mt-2 mb-2"></i> Check Result</h3>
                                </div>
                                <div id="error"></div>
                                <!--Body-->
                                <div>                                    
                                    <select name="class_name" id="class_name" class="mdb-select colorful-select dropdown-info" required="required">
                                        <option value="" disabled selected>Choose Class</option>
                                        <?php echo fill_class_box($conn); ?>
                                    </select>
                                </div>
                                <div>                                    
                                    <select name="session" id="session" class="mdb-select colorful-select dropdown-info" required="required">
                                        <option value="" disabled selected>Choose Year of Result</option>
                                        <?php echo fill_session_box($conn); ?>
                                    </select>
                                </div>
                                <div>                                    
                                    <select name="term" id="term" class="mdb-select colorful-select dropdown-info" required="required">
                                        <option value="" disabled selected>Choose Term</option>
                                        <?php echo fill_term_box($conn); ?>
                                    </select>
                                </div>
                                
                                <div class="md-form">
                                    <i class="fa fa-user prefix grey-text"></i>
                                    <input type="text" name="student_id" id="student_id" class="form-control" autocomplete="off" required="required">
                                    <label for="student_id">Admission No</label>
                                </div>

                                <div class="md-form">
                                    <i class="fa fa-lock prefix grey-text"></i>
                                    <input type="password" name="password" id="password" class="form-control" required="required">
                                    <label for="password">Password</label>
                                </div>
                                <div class="text-center">
                                    <button type="submit" name="download_result" id="download_result" class="btn blue-gradient btn-lg">Submit</button>
                                
                                </div>
                            </div>
                        </div>
                    </form>

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The script:

<?php
require('../../fpdf.php');
require('../conn.php');
error_reporting(0);

function clean($data) {
    $data = trim($data);
    $data = stripslashes($data);
    $data = htmlspecialchars($data);
    return $data;
}

$stu_id = clean($_POST['student_id']);
$class = clean($_POST['class_name']);
$session = clean($_POST['session']);
$term = clean($_POST['term']);

//biodata
$biodata = $conn->prepare("SELECT * FROM ... WHERE Student_ID = ? ");
$biodata->bind_param("s", $stu_id);
$biodata->execute();
$report = $biodata->get_result();
$student = $report->fetch_object();

//date in yyyy-mm-dd format; or it can be in other formats as well
  $birthDate = $student->Date_Of_Birth;
  //explode the date to get month, day and year
  $birthDate = explode("-", $birthDate);
  //get age from date or birthdate
  $age = (date("md", date("U", mktime(0, 0, 0, $birthDate[2], $birthDate[1], $birthDate[0]))) > date("md") ? ((date("Y") - $birthDate[0]) - 1) : (date("Y") - $birthDate[0]));
  $age_year = "".$age." years";

//attendance and comment
$attendance_comment = $conn->prepare("SELECT ... FROM ... LEFT JOIN ... ON ... WHERE Student_ID = ? AND Class_Name = ? AND Session = ? AND Term = ?");

//and so on

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Please help.