morebits: more robust failure handling in wiki.api and wiki.page

To answer the comment on line 2570/2606, which I am removing, we certainly do need the failure callbacks.
They are crucial for tracking the completion of individual tasks in a batch operation.
Without failure callbacks, a failing task looks the same as one that hasn't completed yet, meaning the batch operation can hang if there are too many individual failures.

As for wiki.api's error handling, I think it's fine; UncleDouggie wrote that comment back in e8cc3724577fafe97c9c4a5ec6b5ce48e5c48f70, and only he knows what he had in mind for wiki.api.

Also fixing a long-standing bug - "invalid title" errors were never caught properly.