Fail all ChannelMonitorUpdates after holder_tx_signed

If we've signed the latest holder tx (i.e. we've force-closed and
broadcasted our state), there's not much reason to accept
counterparty-transaction-updating `ChannelMonitorUpdate`s, we
should make sure the `ChannelManager` fails the channel as soon as
possible.

This standardizes the failure cases to also match those added to
the previous commit, which makes things a bit more readable.

Author: TheBlueMatt

lightning/src/chain/channelmonitor.rs

@@ -3286,7 +3286,7 @@ impl<Signer: EcdsaChannelSigner> ChannelMonitorImpl<Signer> {
 			}
 		}
 
-		if ret.is_ok() && (self.funding_spend_seen || self.lockdown_from_offchain) && is_pre_close_update {
+		if ret.is_ok() && (self.funding_spend_seen || self.lockdown_from_offchain || self.holder_tx_signed) && is_pre_close_update {
 			log_error!(logger, "Refusing Channel Monitor Update as counterparty attempted to update commitment after funding was spent");
 			Err(())
 		} else { ret }