Filter disabled publishers on /3.0/content_groups, /3.0/content_groups/:id and /3.0/content_groups/:id
/content
Improve error handling
Merge pull request #218 from atlasapi/mbst-4306-content-group-filter-disabled-publishers

Filter disabled publishers on /3.0/content_groups, /3.0/content_groups/:id and /3.0/content_groups/:id