avoid internal error message when img uses just-deleted page

avoid internal error message when img uses just-deleted page

I think this used to be a fatal error, not just inline error, so I don't
know why it was never noticed, but if a page that an img directive mentions
gets deleted, bestlink() returns a file that no longer exists, and
srcfile() throws an error.

Note that bestlink's behavior of returning a deleted file could be
considered buggy. But, if it's changed to not do that, the page with the img
on it is not updated at all when the file is removed.