using loadexception event.

syntax: store.loadexception() : Fires if an exception occurs in the Proxy during loading.

use beforeload : ( Store this, Object options ) : Fires before a request is made for a new data object. If the beforeload handler returns false the load action will be canceled.

syntax:

store.on('loadexception', function(event, options, response, error) {

alert("Handling the error");

event.stopEvent();

});