channeldb: let AddrsForNode indicate if the node was found or not

Before this commit, the `(channeldb.DB).AddrsForNode` method treats the
results from the channel db and the graph db slightly differently. It
errors out if the channel db is unaware of the node in question but does
not error out if the graph is unaware of the node. So this commit
changes the logic slightly so that a "node-unknown" error from either
backing source is not seen as an error.

chanbackup/backup_test.go

@@ -62,20 +62,19 @@ func (m *mockChannelSource) addAddrsForNode(nodePub *btcec.PublicKey, addrs []ne
 	m.addrs[nodeKey] = addrs
 }
 
-func (m *mockChannelSource) AddrsForNode(nodePub *btcec.PublicKey) ([]net.Addr, error) {
+func (m *mockChannelSource) AddrsForNode(nodePub *btcec.PublicKey) (bool,
+	[]net.Addr, error) {
+
 	if m.failQuery {
-		return nil, fmt.Errorf("fail")
+		return false, nil, fmt.Errorf("fail")
 	}
 
 	var nodeKey [33]byte
 	copy(nodeKey[:], nodePub.SerializeCompressed())
 
 	addrs, ok := m.addrs[nodeKey]
-	if !ok {
-		return nil, fmt.Errorf("can't find addr")
-	}
 
-	return addrs, nil
+	return ok, addrs, nil
 }
 
 // TestFetchBackupForChan tests that we're able to construct a single channel