ENH: capture and handle CanceledError
ENH: capture and handle CanceledError

When `cancel` is called on the task wrapping `_run` capture it.  This
means that the custom exceptions `FailedPause` and `RequestAbort` will
make it down to the plans.  If there is not currently an exception
stashed in `self._exception` put the `CancledException` there.

ericdill/bluesky,ericdill/bluesky