Don’t stop the batch when encountering an invalid title

If the user accidentally specifies an invalid title – for example, one
where non-ASCII characters are URL-encoded – we should skip that page
and still run the rest of the batch, as we do for missing and protected
pages, instead of crashing with a KeyError. This bug was discovered by
User:Sukkoria [1] in batch #328 [2].

[1]: https://fr.wikipedia.org/wiki/Utilisateur:Sukkoria
[2]: https://tools.wmflabs.org/quickcategories/batch/328/