rpcserver: Clarify failure closing offline channel

rpcserver.go

@@ -1071,6 +1071,16 @@ func (r *rpcServer) CloseChannel(in *lnrpc.CloseChannelRequest,
 				}
 			})
 	} else {
+		// If the link is not known by the switch, we cannot gracefully close
+		// the channel.
+		channelID := lnwire.NewChanIDFromOutPoint(chanPoint)
+		if _, err := r.server.htlcSwitch.GetLink(channelID); err != nil {
+			rpcsLog.Debugf("Trying to non-force close offline channel with "+
+				"chan_point=%v", chanPoint)
+			return fmt.Errorf("unable to gracefully close channel while peer "+
+				"is offline (try force closing it instead): %v", err)
+		}
+
 		// Based on the passed fee related parameters, we'll determine
 		// an appropriate fee rate for the cooperative closure
 		// transaction.