fix override aggs bug and delete_slice impl.
fix override aggs bug and delete_slice impl.

Fixed an error when no override aggregates were specified. Improved
delete_slice by no long throwing an exception, but instead returning a
descriptive string which is printed by omni.

ahelsing/geni-tools,tcmitchell/geni-tools,GENI-NSF/gram,tcmitchell/geni-tools,GENI-NSF/gram,plantigrade/geni-tools,plantigrade/geni-tools,GENI-NSF/gram,ahelsing/geni-tools