Recover from HDFS FileSystem handle being closed

I recently saw a bunch of "java.io.IOException: Filesystem closed" messages in the Azkaban web server log, and I think they may be due to having two Reportal instances trying to clean up the same HDFS directory.  I've added some code to catch the IOException thrown when the FileSystem is closed and to do some clean-up, so that a new filesystem handle will be refetched on the next filesystem operation.