Bug fixes, general improvements

* Incorporate Edits tool now deletes route & itinerary for any bus routes or highway projects whose underlying links have all been removed or have had their ANODE/BNODE values modified. Previously, the tool would error out mysteriously when such a case was encountered (extremely rare).
* Import Highway Projects tool now checks REP_ANODE and REP_BNODE values against MHN baselinks to ensure their validity.
* `MHN.die()` can now be called without a message argument.