LSPS2: Fail intercepted htlcs if lsp open channel fails for whatever reason

There's a scenario where we emit `OpenChannel` event to the LSP to handle.  From there `liquidity` is waiting for them to feed us a `ChannelReady` event but if they refuse or fail to open the channel then this HTLC is just stuck forever.

We should expose a method the LSP can use to tell us to go ahead and fail all the pending payments backwards.