fix a NPE when loading a composite P2 repository with no children


git-svn-id: https://svn.apache.org/repos/asf/ant/ivy/core/trunk@1554047 13f79535-47bb-0310-9956-ffa450edef68
Only try to load once a repository which fails to load


git-svn-id: https://svn.apache.org/repos/asf/ant/ivy/core/trunk@1554048 13f79535-47bb-0310-9956-ffa450edef68
Add logs about the downloaded repository metadata


git-svn-id: https://svn.apache.org/repos/asf/ant/ivy/core/trunk@1554049 13f79535-47bb-0310-9956-ffa450edef68