htlcswitch: fix returned failure for insufficient balance

In the scenario where the requested channel does not have enough balance
and another channel towards the same node generates a different failure,
we erroneously returned UnknownNextPeer instead of the expected
TemporaryChannelFailure.

This commit rewrites the non-strict forwarding logic in the switch to
return the proper failure message. Part of this is moving the link
balance check inside the link.

htlcswitch/link.go

@@ -2341,6 +2341,15 @@ func (l *channelLink) canSendHtlc(policy ForwardingPolicy,
 		return &lnwire.FailExpiryTooFar{}
 	}
 
+	// Check to see if there is enough balance in this channel.
+	if amt > l.Bandwidth() {
+		return l.createFailureWithUpdate(
+			func(upd *lnwire.ChannelUpdate) lnwire.FailureMessage {
+				return lnwire.NewTemporaryChannelFailure(upd)
+			},
+		)
+	}
+
 	return nil
 }