[lua] change "failwith" to "error" for consistent error reporting

src/generators/genlua.ml

@@ -1882,7 +1882,7 @@ let transform_multireturn ctx = function
                         e
                     | TReturn Some(e2) ->
                         if is_multireturn e2.etype then
-                            failwith "You cannot return a multireturn type from a haxe function"
+                            error "You cannot return a multireturn type from a haxe function" e2.epos
                         else
                             Type.map_expr loop e;
      (*