QPID-4558: C++ broker selectors: Improved error reporting

git-svn-id: https://svn.apache.org/repos/asf/qpid/trunk@1458610 13f79535-47bb-0310-9956-ffa450edef68
NO-JIRA: silence irrelevant warning in release mode

git-svn-id: https://svn.apache.org/repos/asf/qpid/trunk@1458620 13f79535-47bb-0310-9956-ffa450edef68