Avoid returning user error when stfu is not ready to be sent yet

When we propose quiescence due to a user initiated action, such as a
splice, it's possible that the `stfu` message is not ready to be sent
out yet due to the state machine pending a change. This would result in
an error communicated back to the caller of
`ChannelManager::splice_channel`, but this is unnecessary and confusing,
the splice is still pending and will be carried out once quiescence is
eventually negotiated.

Backport of c2b15c2

Author: wpaulino

lightning/src/ln/channel.rs

@@ -13120,7 +13120,13 @@ where
 
 		self.context.channel_state.set_awaiting_quiescence();
 		if self.context.is_live() {
-			Ok(Some(self.send_stfu(logger)?))
+			match self.send_stfu(logger) {
+				Ok(stfu) => Ok(Some(stfu)),
+				Err(e) => {
+					log_debug!(logger, "{e}");
+					Ok(None)
+				},
+			}
 		} else {
 			log_debug!(logger, "Waiting for peer reconnection to send stfu");
 			Ok(None)