Fix http://issues.apache.org/bugzilla/show_bug.cgi?id=56724
Write an error message to Tomcat logs if container background thread exits unexpectedly.

git-svn-id: https://svn.apache.org/repos/asf/tomcat/trunk@1611506 13f79535-47bb-0310-9956-ffa450edef68
Fix regression with r1611506: NPE on stop (only threadStop should set thread to null, or it needs a sync).

git-svn-id: https://svn.apache.org/repos/asf/tomcat/trunk@1611551 13f79535-47bb-0310-9956-ffa450edef68