Пример отображения ошибок при выводе данных. Попробуйте поискать по полю first_name.
<?php

class jqExceptionOutput extends jqGrid
{
    protected function init()
    {
        #Set database table
        $this->table = 'tbl_customer';

        #Make all columns editable by default
        $this->cols_default = array('editable' => true);

        #Set columns
        $this->cols = array(

            'id' => array('label' => 'ID',
                'width' => 10,
                'align' => 'center',
                'editable' => false, //id is non-editable
            ),

            'first_name' => array('label' => 'First name',
                'width' => 35,
                'editrules' => array('required' => true),
                'formoptions' => array('suffix' => 'gg'),
            ),

            'last_name' => array('label' => 'Last name',
                'width' => 35,
                'editrules' => array('required' => true),
            ),

            'email' => array('label' => 'Email',
                'width' => 30,
                'editrules' => array('email' => true),
            ),

            'phone' => array('label' => 'Phone',
                'width' => 25,
                'align' => 'center',
            ),

            'discount' => array('label' => 'Discount',
                'width' => 15,
                'formatter' => 'numeric',
                'align' => 'center',
                'editable' => false,
            ),
        );

        #Set nav
        $this->nav = array('add' => true, 'edit' => false, 'del' => false);
    }

    protected function search()
    {
        if($this->input['first_name'])
        {
            throw new jqGrid_Exception('You cannot use filter for field "first_name"');
        }

        parent::search();
    }
}
<script>
    <?= $rendered_grid ?>
    $grid.filterToolbar();
</script>