Do not catch GeneratorExit exception

On buildbot 0.8.8 in rare conditions, we hit:

File "/data/buildbot/twisted/internet/threads.py", line 49, in deferToThreadPool
      threadpool.callInThreadWithCallback(onResult, f, *args, **kwargs)
    File "/data/buildbot/twisted/python/threadpool.py", line 148, in callInThreadWithCallb
ack
      self.q.put(o)
    File "//usr/lib/python2.7/Queue.py", line 138, in put
      self.not_empty.notify()
    File "//usr/lib/python2.7/threading.py", line 392, in notify
      for waiter in waiters:
  --- <exception caught here> ---
    File "/data/buildbot/buildbot/process/buildstep.py", line 605, in _startStep_3
      result = yield defer.maybeDeferred(self.start)
  exceptions.GeneratorExit:

this applies the fix from http://trac.buildbot.net/ticket/2395#comment:10 to master/buildbot/process/buildstep.py