routing/payment_lifecycle: return recorded errors

In preparation for MPP we return the terminal errors recorded with the
control tower. The reason is that we cannot return immediately when a
shard fails for MPP, since there might be more shards in flight that we
must wait for. For that reason we instead mark the payment failed in the
control tower, then return this error when we inspect the payment,
seeing it has been failed and there are no shards in flight.

routing/errors.go

@@ -15,10 +15,6 @@ const (
 	// this update can't bring us something new, or because a node
 	// announcement was given for node not found in any channel.
 	ErrIgnored
-
-	// ErrPaymentAttemptTimeout is an error that indicates that a payment
-	// attempt timed out before we were able to successfully route an HTLC.
-	ErrPaymentAttemptTimeout
 )
 
 // routerError is a structure that represent the error inside the routing package,