1044596: Make release listing handle empty data

Add test cases for content path parsing.
1044596: handle http,socket,ssl fetching release

The ContentConnection object doesn't do much
with http errors and the like, and the code in
the cli and gui preferences doesn't expect
exceptions, so handle them here.
Merge pull request #887 from candlepin/alikins/1044596_release_fix

Make release listing handle empty data better.