better cancelation messaging

0.8.3

• Added informative messages as to the root cause of a
  TaskCancelation, e.g.
  "TaskInstance 'doStuff' was canceled because .cancel() was explicitly called"
  "TaskInstance 'doStuff' was canceled because .cancelAll() was explicitly called on the Task"
  "TaskInstance 'doStuff' was canceled because it belongs to a 'restartable' Task that was .perform()ed again"