multi: add custom error messages to channel acceptor

This commit adds an optional error message to the channel acceptor's
reponse to allow operators to inform (or insult) unsuccessful channel
initiators as to the reason for their rejection.

This field is added in addition to the existing accept field to maintain
backwards compatibity. If we were to deprecate accept and interpret a
non-nil error as rejecting the channel, then received a response with
accept=false and a nil error, the server cannot tell whether this is a
legacy rejection or new mesage type acceptance (due to nil error),
so we keep both fields.

chanacceptor/rpcacceptor_test.go

@@ -0,0 +1,85 @@
+package chanacceptor
+
+import (
+	"errors"
+	"strings"
+	"testing"
+
+	"github.com/lightningnetwork/lnd/lnrpc"
+	"github.com/stretchr/testify/require"
+)
+
+// TestValidateAcceptorResponse test validation of acceptor responses.
+func TestValidateAcceptorResponse(t *testing.T) {
+	customError := errors.New("custom error")
+
+	tests := []struct {
+		name        string
+		response    lnrpc.ChannelAcceptResponse
+		accept      bool
+		acceptorErr error
+		error       error
+	}{
+		{
+			name: "accepted with error",
+			response: lnrpc.ChannelAcceptResponse{
+				Accept: true,
+				Error:  customError.Error(),
+			},
+			accept:      false,
+			acceptorErr: errChannelRejected,
+			error:       errAcceptWithError,
+		},
+		{
+			name: "custom error too long",
+			response: lnrpc.ChannelAcceptResponse{
+				Accept: false,
+				Error:  strings.Repeat(" ", maxErrorLength+1),
+			},
+			accept:      false,
+			acceptorErr: errChannelRejected,
+			error:       errCustomLength,
+		},
+		{
+			name: "accepted",
+			response: lnrpc.ChannelAcceptResponse{
+				Accept: true,
+			},
+			accept:      true,
+			acceptorErr: nil,
+			error:       nil,
+		},
+		{
+			name: "rejected with error",
+			response: lnrpc.ChannelAcceptResponse{
+				Accept: false,
+				Error:  customError.Error(),
+			},
+			accept:      false,
+			acceptorErr: customError,
+			error:       nil,
+		},
+		{
+			name: "rejected with no error",
+			response: lnrpc.ChannelAcceptResponse{
+				Accept: false,
+			},
+			accept:      false,
+			acceptorErr: errChannelRejected,
+			error:       nil,
+		},
+	}
+
+	for _, test := range tests {
+		test := test
+
+		t.Run(test.name, func(t *testing.T) {
+			accept, acceptErr, err := validateAcceptorResponse(
+				test.response,
+			)
+			require.Equal(t, test.accept, accept)
+			require.Equal(t, test.acceptorErr, acceptErr)
+			require.Equal(t, test.error, err)
+		})
+	}
+}