/apps/twnlua: add error on attempt to import twnapi.lua

apps/twnlua/docgen.py

@@ -31,6 +31,8 @@ def to_lua_type_annot(typedesc):
         return "unknown"
         # raise BaseException("Unhandled type for annotation: %s" % typedesc)
 
+print("error(\"townengine lua api file is not supposed to be imported!\")")
+
 type_annotations, enum_annotations = {}, {}
 type_annotations["ctx"] = r"{ %s, udata: table }" % \
     ', '.join("%s: %s" % (f["name"], to_lua_type_annot(f["type"])) for f in api["types"]["Context"]["fields"])