When visiting Special:GoToLinkedPage and the given item does not have a sitelink for the given site it just shows the form. We should at least notify the user that there is no such page at the moment and maybe also link them to reasonator and/or add a lnk to the item's page. There should also be error messages when an invalid or non-existent id was given and so on.
Whiteboard: u=dev c=backend p=0
T72159: Forward according to user's preferred language on Special:GoToLinkedPage