multi: coop close with active HTLCs on the channel

For the lncli cmd we now always initiate the coop close even if
there are active HTLCs on the channel. In case HTLCs are on the
channel and the coop close is initiated LND handles the closing
flow in the background and the lncli cmd will block until the
transaction is broadcasted to the mempool. In the background LND
disallows any new HTLCs and waits until all HTLCs are resolved
before kicking of the negotiation process.
Moreover if active HTLCs are present and the no_wait param is not
set the error msg is now highlightning it so the user can react
accordingly.

itest/lnd_coop_close_with_htlcs_test.go

@@ -93,14 +93,17 @@ func coopCloseWithHTLCs(ht *lntest.HarnessTest) {
 	// closure is set up. Let's settle the invoice.
 	alice.RPC.SettleInvoice(preimage[:])
 
-	// Pull the instant update off the wire to clear the path for the
-	// close pending update.
-	_, err := closeClient.Recv()
+	// Pull the instant update off the wire and make sure the number of
+	// pending HTLCs is as expected.
+	update, err := closeClient.Recv()
 	require.NoError(ht, err)
+	closeInstant := update.GetCloseInstant()
+	require.NotNil(ht, closeInstant)
+	require.Equal(ht, closeInstant.NumPendingHtlcs, int32(1))
 
 	// Wait for the next channel closure update. Now that we have settled
 	// the only HTLC this should be imminent.
-	update, err := closeClient.Recv()
+	update, err = closeClient.Recv()
 	require.NoError(ht, err)
 
 	// This next update should be a GetClosePending as it should be the