#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=1281
Server was changed to return a BadRequest along with an invalid url error if a content item has a bad url. Media source url is checked as well if present.  Client will need to be changed to display these errors.

This merge also addresses #726

## Original pull request https://github.com/alterm4nn/ChronoZoom/pull/714