show_ajax_error: function(response){
    var msg = '<b>Status: ' + response.status + ' => ' + response.statusText + '</b><br><br>' +
      '<div style="height:400px;overflow:scroll;">' + response.responseText + '<\div>';
    Ext.Msg.show({title: 'Error', msg: msg, minWidth: 900, maxWidth: 900, buttons: Ext.Msg.OK, icon: Ext.Msg.ERROR});
  }