Private posts can only be boosted by explicitly setting boost privacy to private

When boosting through the mastodon REST API (/api/v1/statuses/:id/reblog), private posts can only be boosted if the visibility parameter is explicitly set to private, which is not done by MastoFE or most Mastodon clients. Indeed, in such cases, Mastodon sets the announce visibility to private, but Pleroma returns a 404 error.

I am not sure why this happens, as CommonAPI.repeat should be handling that case through the public_announce? helper, but obviously something goes wrong.

Additionally, it would be nice if the error returned by Pleroma in case the visibility is invalid would not be a misleading 404 error.