ChannelManager::force_close_channel does not fail on random channel_id

[sr-gi]

ChannelManager::force_close_channel attempts to close any given channel_id, even if the latter is unknown by the manager.

This contrasts with ChannelManager::close_channel, where a errors::APIError::ChannelUnavailable will be raised.

Having the same interface regarding unknown channels by both methods feels like the way to go, so I think force_close_channel should be updated.

[sr-gi]

I can tackle this issue if there's consensus on the update btw.

[TheBlueMatt]

I can tackle this issue if there's consensus on the update btw.

I'd say go for it.