Fix the err msg provided when a send fails due to peer disconnected

We haven't had a `MonitorUpdateInProgress` check in
`Channel::is_live` for some time.

Author: TheBlueMatt

fuzz/src/chanmon_consistency.rs

@@ -295,7 +295,7 @@ fn check_api_err(api_err: APIError) {
 			// all others. If you hit this panic, the list of acceptable errors
 			// is probably just stale and you should add new messages here.
 			match err.as_str() {
-				"Peer for first hop currently disconnected/pending monitor update!" => {},
+				"Peer for first hop currently disconnected" => {},
 				_ if err.starts_with("Cannot push more than their max accepted HTLCs ") => {},
 				_ if err.starts_with("Cannot send value that would put us over the max HTLC value in flight our peer will accept ") => {},
 				_ if err.starts_with("Cannot send value that would put our balance under counterparty-announced channel reserve value") => {},

lightning/src/ln/channelmanager.rs

@@ -2390,10 +2390,10 @@ where
 			let mut peer_state_lock = peer_state_mutex.lock().unwrap();
 			let peer_state = &mut *peer_state_lock;
 			if let hash_map::Entry::Occupied(mut chan) = peer_state.channel_by_id.entry(id) {
+				if !chan.get().is_live() {
+					return Err(APIError::ChannelUnavailable{err: "Peer for first hop currently disconnected".to_owned()});
+				}
 				match {
-					if !chan.get().is_live() {
-						return Err(APIError::ChannelUnavailable{err: "Peer for first hop currently disconnected/pending monitor update!".to_owned()});
-					}
 					break_chan_entry!(self, chan.get_mut().send_htlc_and_commit(
 						htlc_msat, payment_hash.clone(), htlc_cltv, HTLCSource::OutboundRoute {
 							path: path.clone(),

lightning/src/ln/payment_tests.rs

@@ -258,7 +258,7 @@ fn no_pending_leak_on_initial_send_failure() {
 
 	unwrap_send_err!(nodes[0].node.send_payment(&route, payment_hash, &Some(payment_secret), PaymentId(payment_hash.0)),
 		true, APIError::ChannelUnavailable { ref err },
-		assert_eq!(err, "Peer for first hop currently disconnected/pending monitor update!"));
+		assert_eq!(err, "Peer for first hop currently disconnected"));
 
 	assert!(!nodes[0].node.has_pending_payments());
 }