Wordpress - Create own admin messages for Custom Post Type - wordpress

Question

I've created a custom post type called routes and I'd like to be able to return error messages to the screen when something goes wrong during a save/update e.g. The type allows for gpx/kml files to be uploaded and checked that the correct type has been posted. At the moment it just returns if it goes wrong - how can I set an error message?
//Return if file type wrong.
if($file_type != 'application/octet-stream' && $file_type != 'application/gpx+xml' ) {
return;
}

Answer 1

try this
example:
add_admin_message('Please enter valid URL for the project link', true);
add_admin_message('Your custom post type was updated');
source:
<?php
/**
* Messages with the default wordpress classes
*/
function showMessage($message, $errormsg = false)
{
if ($errormsg) {
echo '<div id="message" class="error">';
}
else {
echo '<div id="message" class="updated fade">';
}
echo "<p>$message</p></div>";
}
/**
* Display custom messages
*/
function show_admin_messages()
{
if(isset($_COOKIE['wp-admin-messages-normal'])) {
$messages = strtok($_COOKIE['wp-admin-messages-normal'], "##");
while ($messages !== false) {
showMessage($messages, true);
$messages = strtok("##");
}
setcookie('wp-admin-messages-normal', null);
}
if(isset($_COOKIE['wp-admin-messages-error'])) {
$messages = strtok($_COOKIE['wp-admin-messages-error'], "##");
while ($messages !== false) {
showMessage($messages, true);
$messages = strtok("##");
}
setcookie('wp-admin-messages-error', null);
}
}
/**
* Hook into admin notices
*/
add_action('admin_notices', 'show_admin_messages');
/**
* User Wrapper
*/
function add_admin_message($message, $error = false)
{
if(empty($message)) return false;
if($error) {
setcookie('wp-admin-messages-error', $_COOKIE['wp-admin-messages-error'] . '##' . $message, time()+60);
} else {
setcookie('wp-admin-messages-normal', $_COOKIE['wp-admin-messages-normal'] . '##' . $message, time()+60);
}
}