In the zookeeper jobboard (and persistence backends) we
are associating the cause of a new exception when raising a
exception. Using the new exception helper we can make this
work better on py2.x and py3.x so that the py3.x version
has the chain setup correctly (while the py2.x version just
uses the class 'cause' attribute instead).
Change-Id: Ieeac2f70e1834d4612556565762ffd3be3e5b5a1