Reporting errors in a shortcode plugin

Take a look at admin notices. If you the cols parameter isn’t set use some code like this:

function sample_admin_notice_fail() {
    ?>
    <div class="notice notice-error is-dismissible">
        <p><?php _e( 'You forgot the cols parameter!', 'sample-text-domain' ); ?></p>
    </div>
    <?php
}

add_action( 'admin_notices','sample_admin_notice_fail' );

to display this notice:

on top of your admin page.

You find some more information about implementing admin notices on this page.