routing: return err on validation failure of network update
routing: return err on validation failure of network update

This ensures the waiting receiving channel always receives an error to
prevent a deadlock when processing a network update that fails due to
the validation barrier.

On commit d5aedbcbd9db510c974c9f7be5ab177ad6546994:

1000 @ 0x43a285 0x44a38f 0xc42e86 0xc80fda 0xc8682d 0xc976c9 0x46fce1
github.com/lightningnetwork/lnd/routing.(*ChannelRouter).AddNode+0x245                      github.com/lightningnetwork/lnd/routing/router.go:2218
github.com/lightningnetwork/lnd/discovery.(*AuthenticatedGossiper).addNode+0x3b9            github.com/lightningnetwork/lnd/discovery/gossiper.go:1510
github.com/lightningnetwork/lnd/discovery.(*AuthenticatedGossiper).processNetworkAnnouncement+0x574c github.com/lightningnetwork/lnd/discovery/gossiper.go:1554
github.com/lightningnetwork/lnd/discovery.(*AuthenticatedGossiper).networkHandler.func1+0x24github.com/lightningnetwork/lnd/discovery/gossiper.go:1043

aakselrod/lnd,aakselrod/lnd