Added the vnErrorModalService to show the server error modal

@hippee-lee @tsanko @kevinsproles please review the PR. This includes the addition of vnErrorModalService which exposes a method to open the )modal window (angular-ui/boostrap) with a template url passed in, or if nothing is passed in uses a default template. This service will be invoked from the theme on the `VN_HTTP_500_ERROR` event on the $rootScope.