Fix katello check when candlepin fixes -> bz 1207822
Candlepin doesnt raise the proper error message when duplicate content id is added to an environment.
This has lead to issues with https://github.com/Katello/katello/pull/5155/files#diff-b6e97a4a6ef35bf17dcf3b55c836bce1R42
where we act upon any 500 error and assume its due to some duplicate content id error.
This issue is a place holder to fix that when cp fixes bz 1207822