Fix arity and return value mismatches

`assert` returns every argument it is given, so `return assert(x, msg)`
was leaking the message as a second return value.

`get_tile` is documented as taking x and y. The async function class
declared `@operator call` with no parameters, which made every async
call look over supplied.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

exp_legacy/module/expcore/roles.lua

@@ -376,7 +376,7 @@ function Roles.get_player_highest_role(player)
         end
     end
 
-    return assert(highest, "Player has no roles")
+    return (assert(highest, "Player has no roles"))
 end
 
 --- Assignment.