PHP passing messages back to Ajax

I have removed a lot of the code, but you should get an idea of what I am doing:

<?php

$upOne = realpath(__DIR__ . '/..');
require_once $upOne . '/vendor/autoload.php';

use phpseclib\Net\SFTP;

$errors = array();
$fileData = array();
$inputArray = array();
$config = new CONFIG\Config();


if (empty($_POST['emailAdd'])) {
    $errors['emailAdd'] = '- Please input your Email address';
} else {
    if (!filter_var($_POST['emailAdd'], FILTER_VALIDATE_EMAIL)) {
        $errors['emailAdd'] = '- Please input your Email address';
    } else {
        $email = filter_var($_POST['emailAdd'], FILTER_SANITIZE_EMAIL);
        array_push($inputArray, $email);
    }
}

//Additional validation
...

if (!empty($errors)) {
    echo json_encode("failed");
} else {
    try {
        $dbh = new PDO("mysql:host=".$config::DB_HOST.";dbname=".$config::DB_NAME."", $config::DB_USER, $config::DB_PASSWORD);

        $stmt = $dbh->prepare("INSERT INTO emirates_user("...");

        if($stmt->execute()) {

            $lastId = $dbh->lastInsertId();

            if (isset($_FILES)) {
                foreach($_FILES as $file) {
                    $upload = UPLOAD\Upload::factory($config::TEMP_DIR);
                    $upload->file($file);

                    if(!$upload->get_errors()) {
                        $fileData[$results['full_path']] = $results['mime'];
                    }
                    else {
                        echo json_encode("failed");
                        break;
                    }
                }

                $date = date('_dmY_H:i:s');
                $pdf = new PDF\PdfGenerator($fileData, $inputArray, $date);

                if($pdf) {
                    $command = '/usr/bin/python /srv/www/vhosts/someurl/html/php/somefile.py';
                    exec($command, $retval);

                    if(isset($retval[0])  && $retval[0] === 'success') {
                        $mail = new PHPMailer;

                        .....

                        if(!$mail->send()) {
                            echo json_encode("failed");
                        } else {
                            $lstmt = $dbh->prepare("...");
                            ...

                            if($lstmt->execute()) {
                                echo json_encode("success");
                            }
                        }
                    } else {
                        echo json_encode("failed");
                    }
                }

            } else {
                echo json_encode("failed");
            }
        } else {
            echo json_encode("failed");
        }
    }
    catch(Exception $e) {
        echo json_encode("failed");
    }
}

First of all, I validate the user input. If this fails, I return a failed message back to my Ajax. If all is ok, the process begins. This script inserts into a database, generates a PDF, SFTP's the file using a Python script, sends an email etc. All of these actions could potentially go wrong. I have handled things via conditional statements to make sure the code is executed correctly. If not, then I return a failed message again. If it makes it to the very end, I return a success message.

My Ajax is like so (not the whole function, just the done and failure sections):

.done(function (response) {
    if(response == 'failed'){
        window.location.replace('failure.html');
    } else {
        window.location.replace('success.html');
    }
}).fail(function (jqXHR, textStatus) {
    console.log(textStatus);
});

So if the response is failed, I display a failed page.

Really, I am just seeking advice as to whether this type of logic is ok. Should I maybe return more detail information about the error so I know what it relates too? If so, how would I handle this in Ajax?