Better error handling for adding applications without participants

[zopyx]

Adding an application without defining a participants leads to a Python-level error message.
Some better error checking and a more app-level error message would be nice here:

ajung@dev ~/src/async-testbed/shoobx.wfmc (master) $ python cycle.py 
ProcessStarted(Process('sample'))
Transition(None, Activity('sample.start'))
ActivityStarted(Activity('sample.start'))
ActivityFinished(Activity('sample.start'))
Traceback (most recent call last):
  File "cycle.py", line 51, in <module>
    proc.start()
  File "/home/ajung/.pyenv/versions/2.7.14/lib/python2.7/site-packages/shoobx/wfmc/process.py", line 698, in start
    self.transition(None, (self.startTransition, ))
  File "/home/ajung/.pyenv/versions/2.7.14/lib/python2.7/site-packages/shoobx/wfmc/process.py", line 777, in transition
    next.start(transition)
  File "/home/ajung/.pyenv/versions/2.7.14/lib/python2.7/site-packages/shoobx/wfmc/process.py", line 467, in start
    self.finish()
  File "/home/ajung/.pyenv/versions/2.7.14/lib/python2.7/site-packages/shoobx/wfmc/process.py", line 529, in finish
    self.process.transition(self, transitions)
  File "/home/ajung/.pyenv/versions/2.7.14/lib/python2.7/site-packages/shoobx/wfmc/process.py", line 773, in transition
    next = self.ActivityFactory(self, activity_definition)
  File "/home/ajung/.pyenv/versions/2.7.14/lib/python2.7/site-packages/shoobx/wfmc/process.py", line 349, in __init__
    self.createWorkItems()
  File "/home/ajung/.pyenv/versions/2.7.14/lib/python2.7/site-packages/shoobx/wfmc/process.py", line 376, in createWorkItems
    for workitem, application, formal, actual in workitems:
  File "/home/ajung/.pyenv/versions/2.7.14/lib/python2.7/site-packages/shoobx/wfmc/process.py", line 387, in createApplicationWorkItems
    participant = integration.createParticipant(
AttributeError: 'NoneType' object has no attribute 'createParticipant'