Unlisted boards cause an error after the NSFW check

[loynet]

The board whitelist allows one to specify a list of boards to be checked and, at the moment, some of them may be secret (unlisted). The method post_thread checks if a board is NSFW by fetching information in the <jschanib.tld>/boards.json endpoint but this endpoint only returns info about listed boards.

Maybe we should only allow listed boards on the whitelist? If there's no way to know if an unlisted board is NSFW we can't mark its media appropriately.

[Lacerkage]

Does content published on unlisted boards appear on the overboard?
If not, then I suggest we should ignore unlisted boards for the moment