fix(util): throw error when au property is not foun on an element
fix(util): throw error when au property is not foun on an element

closes https://github.com/aurelia-ui-toolkits/aurelia-kendoui-bridge/issues/598

aurelia-ui-toolkits/aurelia-kendoui-bridge,aurelia-ui-toolkits/aurelia-kendoui-plugin,aurelia-ui-toolkits/aurelia-kendoui-bridge,aurelia-ui-toolkits/aurelia-kendoui-plugin