#605 Authoring: User get unclear error message when tries to create artifact with incorrect image url

Bug #605 Authoring: User get unclear error message when tries to create artifact with incorrect image url
http://mrccodereview.cloudapp.net/ui#review:id=1279
Server was changed to return a BadRequest along with an invalid url error if a content item has a bad url.  Client will need to be changed to display this error.