Observer throw Exception message without loading logic

Question

Observer :

public function cartevent(Varien_Event_Observer $observer) {

        // Load product
        $product = $observer->getProduct();
        $prodID = $product->getId();
        $_product = Mage::getModel('catalog/product')->load($prodID);

        // get category id
        $categoryIds = $_product->getCategoryIds();

        // check cart
        $cart_qty = (int)Mage::getModel('checkout/cart')->getQuote()->getItemQty();

        // check conditiion cart has other products 
        if(in_array(681, $categoryIds) && $cart_qty > 0) {
            Mage::throwException("You can not add This special Product, empty cart before add it");
        }

        // check if other products add to cart
        $quote = Mage::getModel('checkout/cart')->getQuote();
        foreach ($quote->getAllItems() as $item) {

            $_product1 = $item->getProduct();
            $categoryIds1 = $_product1->getCategoryIds();
            if(in_array(681, $categoryIds1)) {
                Mage::throwException("If you want Kolu Padi, Purchase alone not mixed with other Products");
                break;
            }
        }
    }  

If try to add cart in website, gettin error,

a:5:{i:0;s:67:"If you want Kolu Padi, Purchase alone not mixed with other Products";i:1;s:1835:"#0 /home/abc/public_html/app/code/local/Gta/KolupadiRestrict/Model/Observer.php(21): Mage::throwException('If you want Kol...')
#1 /home/abc/public_html/app/code/core/Mage/Core/Model/App.php(1358): Gta_KolupadiRestrict_Model_Observer->cartevent(Object(Varien_Event_Observer))
#2 /home/abc/public_html/app/code/core/Mage/Core/Model/App.php(1337): Mage_Core_Model_App->_callObserverMethod(Object(Gta_KolupadiRestrict_Model_Observer), 'cartevent', Object(Varien_Event_Observer))
#3 /home/abc/public_html/app/Mage.php(448): Mage_Core_Model_App->dispatchEvent('checkout_cart_p...', Array)
#4 /home/abc/public_html/app/code/core/Mage/Checkout/Model/Cart.php(290): Mage::dispatchEvent('checkout_cart_p...', Array)
#5 /home/abc/public_html/app/code/local/Cmsmart/AjaxCart/controllers/IndexController.php(315): Mage_Checkout_Model_Cart->addProduct('5071', Array)
#6 /home/abc/public_html/app/code/local/Cmsmart/AjaxCart/controllers/IndexController.php(133): Cmsmart_AjaxCart_IndexController->tryaddAction(Object(Mage_Catalog_Model_Product), Array)
#7 /home/abc/public_html/app/code/core/Mage/Core/Controller/Varien/Action.php(418): Cmsmart_AjaxCart_IndexController->indexAction()
#8 /home/abc/public_html/app/code/core/Mage/Core/Controller/Varien/Router/Standard.php(254): Mage_Core_Controller_Varien_Action->dispatch('index')
#9 /home/abc/public_html/app/code/core/Mage/Core/Controller/Varien/Front.php(172): Mage_Core_Controller_Varien_Router_Standard->match(Object(Mage_Core_Controller_Request_Http))
#10 /home/abc/public_html/app/code/core/Mage/Core/Model/App.php(365): Mage_Core_Controller_Varien_Front->dispatch()
#11 /home/abc/public_html/app/Mage.php(683): Mage_Core_Model_App->run(Array)
#12 /home/abc/public_html/australia/index.php(93): Mage::run('australia', 'website')
#13 {main}";s:3:"url";s:100:"/ajaxcart/index/index/?form_key=MtlJbvKkqPQKCLJR&product=5071&related_product=&qty=1&_=1565155804563";s:11:"script_name";s:10:"/index.php";s:4:"skin";s:9:"australia";}

Answer 1

Please use the below code

 public
    function cartevent(Varien_Event_Observer $observer)
    {

    // Load product
        $product = $observer->getProduct();
        $prodID = $product->getId();
        $_product = Mage::getModel('catalog/product')->load($prodID);

    // get category id
        $categoryIds = $_product->getCategoryIds();

    // check cart
        $cart_qty = (int)Mage::getModel('checkout/cart')->getQuote()->getItemQty();

    // check conditiion cart has other products
        if (in_array(681, $categoryIds) && $cart_qty > 0) {

 Mage::getSingleton('core/session')->addError("You can not add This special Product, empty cart before add it");
                $url = Mage::helper('core/http')->getHttpReferer() ? Mage::helper('core/http')->getHttpReferer() : Mage::getUrl();
                Mage::app()->getFrontController()->getResponse()->setRedirect($url);
                Mage::app()->getResponse()->sendResponse();
                return Mage::app()->getFrontController()->getResponse()->getRedirect($url);;


        }

    // check if other products add to cart
        $quote = Mage::getModel('checkout/cart')->getQuote();
        foreach ($quote->getAllItems() as $item) {
            $prdId = $item->getProductId();
            if ($prdId != $prodID) {
                $_product1 = $item->getProduct();


                $categoryIds1 = $_product1->getCategoryIds();
                if (in_array(681, $categoryIds1)) {

                    Mage::getSingleton('core/session')->addError("If you want Kolu Padi, Purchase alone not mixed with other Products");
                    $url = Mage::helper('core/http')->getHttpReferer() ? Mage::helper('core/http')->getHttpReferer() : Mage::getUrl();
                    Mage::app()->getFrontController()->getResponse()->setRedirect($url);
                    Mage::app()->getResponse()->sendResponse();
                    return Mage::app()->getFrontController()->getResponse()->getRedirect($url);;

                }
            }

        }
    }