warnings should probably be errors for non-existing parents in xdoc topics

[MattKaufmann]

Consider changing warnings "found topics with non-existent parents"
into errors, just as we already treat broken links as errors. A
simple fix might be to replace the following code in the definition of
function save-json-files, in file books/xdoc/save-fancy.lisp.

    (or (not orphans)
        (cw "~|~%WARNING: found topics with non-existent parents:~%~x0~%These ~
             topics may only show up in the index pages.~%~%" orphans))

The new code would presumably be like that code that is near calls of
note-xdoc-error in that same file. This might be trivial, but I'll
leave it till later, and probably to someone else to take on; perhaps
there is some issue I haven't considered.