Add rudimentary error messages to the special nodes parser

- It tells you the line number, but not the nature of the error
- Unfortunately it can't currently cope with files that don't end in a newline

rsrc/boes/town/t0.spec

@@ -10,4 +10,4 @@ def Varn = 3
     msg  2          # Shows string 2; the omitted argument is assumed to be -1
 @town-visible = 5   # Explicitly specify node number
     msg  3,4        # Shows strings 3 and 4 in the message dialog
-    ex1  Varn,1     # Town 3, set visibility to true
+    ex1  Varn,1     # Town 3, set visibility to true