Add `NX_NAME` configuration macro.

Add an `NX_NAME` configuration macro (and a corresponding Lua `nxconfig`
entry), which defines what this campaign is named currently.
Add friendly error message for missing lp8.

Add a friendly error message for when NX is run without an lp8 being
available.

After the message is shown, the scenario is ended in defeat.

At least, I hope it’s friendlier than Lua spewing a backtrace.