Check for linedef actions referring to macro that aren't defined

[thekovic]

Such a linedef will cause the map to fail to load (throws an error message in EX/EX+ and hard crashes in Remaster) but is technically legal in DB64 and doesn't cause any warnings or errors. This makes it hard to determine what the error is if your map is not working (particularly if you're working with the Remaster).

Possible solutions:

1. Add some sort of warning/error mechanism checking for these linedefs.
2. Implicitly define all macros that haven't been manually defined in the Macro Editor as macros with empty bodies. This will hush any problems because the linedef action will no longer be invalid, it will just refer to a valid macro that doesn't do anything.