Added a general comment describing jsonurl.py and updated the error
handling of recursive key lookup to avoid printing the Python "stack
trace" when there is an exception (i.e., more becoming of a utility).

Review: https://reviews.apache.org/r/9376

git-svn-id: https://svn.apache.org/repos/asf/incubator/mesos/trunk@1448446 13f79535-47bb-0310-9956-ffa450edef68