Fix http://issues.apache.org/bugzilla/show_bug.cgi?id=56724
Write an error message to Tomcat logs if container background thread exits unexpectedly.
It is backport of r1611506 from tomcat/trunk.


git-svn-id: https://svn.apache.org/repos/asf/tomcat/tc7.0.x/trunk@1611509 13f79535-47bb-0310-9956-ffa450edef68