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

Server was changed to return a BadRequest along with an invalid url error if a content item has a bad url. Media Source is also checked if present.  Client will need to be changed to display these error.
Merge pull request #735 from xanthics/master

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