WikipediaHandler: ?wikipedia command searches when no article found

[FastLizard4]

When a user calls the ?wikipedia command, the bot should search for the best matching article and return a summary of it (ideally with a note "Couldn't find 'x', but a search found this:" or similar). Note that this shouldn't apply to [[wikilinks]], which should continue to simply return a "no such article" error on non-existent pages.