Wrong behaviour when undo_previous_slices = False

[mhidas]

If we want the handler to only undo the current slice in the event of a harvest failure, it will only undo the failing harvest itself, but not any previous successful harvesters that ran on the same slice. See #23 (comment)

This is only an issue if we have multiple harvesters AND multiple slices AND only want to unharvest the failing slice. Currently I don't know of such a use case, so I don't think this is high priority.

[mhidas]

Unittests will need updating to test this scenario (#34 (comment))