Improve lncli closechannel error messages for mempool fee and channel state (#9155)

lnwallet/btcwallet/btcwallet.go

@@ -1136,7 +1136,7 @@ func mapRpcclientError(err error) error {
 	case errors.Is(err, chain.ErrMempoolMinFeeNotMet),
 		errors.Is(err, chain.ErrMinRelayFeeNotMet):
 
-		return fmt.Errorf("%w: %v", lnwallet.ErrMempoolFee, err.Error())
+		return fmt.Errorf("%w: %v. The cooperative close transaction was created but not accepted into the mempool due to low fees. You may need to use 'lncli wallet bumpfee' to increase the fee and get the transaction confirmed", lnwallet.ErrMempoolFee, err.Error())
 	}
 
 	return err

rpcserver.go

@@ -2849,13 +2849,16 @@ func (r *rpcServer) CloseChannel(in *lnrpc.CloseChannelRequest,
 			// RBF variant, then we can actually bypass the switch.
 			// Otherwise, we'll return an error.
 			if !chanHasRbfCloser {
-				rpcsLog.Debugf("Trying to non-force close "+
-					"offline channel with chan_point=%v",
-					chanPoint)
+			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)
+			// Check if the channel is already closed or has a pending close tx.
+				if channel.HasChanStatus(channeldb.ChanStatusCoopBroadcasted) {
+					return fmt.Errorf("channel is already in the process of closing. Check 'lncli pendingchannels' for status. If the close transaction is not confirmed, you may need to use 'lncli wallet bumpfee'")
+				}
+
+			return fmt.Errorf("unable to gracefully close channel while peer is offline (try force closing it instead): %v", err)
 			}
 		}