Fix error reporting from Suppy.quit

this changes the UDP error on linux from "Unexplained error"
to "Unknown system error", which is not really that much more helpful
(but the fix is still worth it on its own)

src/core/Supply.pm

@@ -1460,7 +1460,7 @@ my class Supplier {
         $!taplist.quit($ex);
     }
     multi method quit(Supplier:D: Str() $message) {
-        $!taplist.quit(X::AdHoc.new(:$message));
+        $!taplist.quit(X::AdHoc.new(payload => $message));
     }
 
     method Supply(Supplier:D:) {