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.

channeldb/db_test.go

@@ -219,7 +219,7 @@ func TestAddrsForNode(t *testing.T) {
 
 	// Now that we've created a link node, as well as a vertex for the
 	// node, we'll query for all its addresses.
-	nodeAddrs, err := fullDB.AddrsForNode(nodePub)
+	_, nodeAddrs, err := fullDB.AddrsForNode(nodePub)
 	require.NoError(t, err, "unable to obtain node addrs")
 
 	expectedAddrs := make(map[string]struct{})