Actually there is a 404 error return for the two following situation :
1. wrong weblink id
2. expired weblink
It could be good to serve a proper page to represent the state off the link.
Note : From my other post (viewtopic.php?f=3&t=3541" rel="nofollow) it seems that we can have a case were weblink is valid but no file is available.
I didn't tested it but maybe a file doesn't exist can be also added.