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/interface.go

@@ -1,10 +1,19 @@
 package chanacceptor
 
 import (
+	"errors"
+
 	"github.com/btcsuite/btcd/btcec"
 	"github.com/lightningnetwork/lnd/lnwire"
 )
 
+var (
+	// errChannelRejected is returned when the rpc channel acceptor rejects
+	// a channel due to acceptor timeout, shutdown, or because no custom
+	// error value is available when the channel was rejected.
+	errChannelRejected = errors.New("channel rejected")
+)
+
 // ChannelAcceptRequest is a struct containing the requesting node's public key
 // along with the lnwire.OpenChannel message that they sent when requesting an
 // inbound channel. This information is provided to each acceptor so that they
@@ -18,8 +27,48 @@ type ChannelAcceptRequest struct {
 	OpenChanMsg *lnwire.OpenChannel
 }
 
-// ChannelAcceptor is an interface that represents a predicate on the data
+// ChannelAcceptResponse is a struct containing the response to a request to
+// open an inbound channel.
+type ChannelAcceptResponse struct {
+	// ChanAcceptError the error returned by the channel acceptor. If the
+	// channel was accepted, this value will be nil.
+	ChanAcceptError
+}
+
+// NewChannelAcceptResponse is a constructor for a channel accept response,
+// which creates a response with an appropriately wrapped error (in the case of
+// a rejection) so that the error will be whitelisted and delivered to the
+// initiating peer. Accepted channels simply return a response containing a nil
+// error.
+func NewChannelAcceptResponse(accept bool,
+	acceptErr error) *ChannelAcceptResponse {
+
+	// If we want to accept the channel, we return a response with a nil
+	// error.
+	if accept {
+		return &ChannelAcceptResponse{}
+	}
+
+	// Use a generic error when no custom error is provided.
+	if acceptErr == nil {
+		acceptErr = errChannelRejected
+	}
+
+	return &ChannelAcceptResponse{
+		ChanAcceptError: ChanAcceptError{
+			error: acceptErr,
+		},
+	}
+}
+
+// RejectChannel returns a boolean that indicates whether we should reject the
+// channel.
+func (c *ChannelAcceptResponse) RejectChannel() bool {
+	return c.error != nil
+}
+
+// ChannelAcceptor is an interface that represents  a predicate on the data
 // contained in ChannelAcceptRequest.
 type ChannelAcceptor interface {
-	Accept(req *ChannelAcceptRequest) bool
+	Accept(req *ChannelAcceptRequest) *ChannelAcceptResponse
 }