script: work around a KeyError exception in Python Wikipedia lib.

Even the test `if hasattr(page, 'links')` would trigger this exception.
So I try the approach "Easier to Ask Forgiveness than Permission".
Weird stuff but well…
Note: I had this exception when running it on the Maltese data.

script/BuildLangModel.py

@@ -257,7 +257,10 @@ def visit_pages(titles, depth, lang, logfd):
         logfd.write("\n{} (revision {})".format(title, page.revision_id))
 
         process_text(page.content, lang)
-        next_titles += page.links
+        try:
+            next_titles += page.links
+        except KeyError:
+            pass
 
     if depth >= options.max_depth:
         return