Make the `ABANDONED` message type available to use in `BaseTask`

[mdickinson]

#392 refactored what's now the BaseTask to add a new ABANDONED message type, used for cancellation. That fixed a situation where we were inventing a bogus return value of None when the task wasn't even being executed.

But individual tasks are still inventing a bogus return value of None in response to cancellation. We could fix up the BaseTask to make it easier to check for cancellation and send the ABANDONED message if cancellation occurred.

Examples of the bogus return value:

• traits-futures/traits_futures/background_iteration.py

  Line 40 in abeb975

  return None

• traits-futures/traits_futures/background_progress.py

  Line 98 in abeb975

  return None