LOG4PHP-178: 
* Updated Logger::setLevel() to accept null.
* Updated LoggerRoot::setLevel() to deny null.
* Added error reporting to LoggerRoot.
* Updated tests.

git-svn-id: https://svn.apache.org/repos/asf/logging/log4php/trunk@1343190 13f79535-47bb-0310-9956-ffa450edef68