htlcswitch+router: define PaymentResult, GetPaymentResult

This lets us distinguish an critical error from a actual payment result
(success or failure). This is important since we know that we can only
attempt another payment when a final result from the previous payment
attempt is received.

htlcswitch/payment_result.go

@@ -0,0 +1,25 @@
+package htlcswitch
+
+import "errors"
+
+var (
+	// ErrPaymentIDNotFound is an error returned if the given paymentID is
+	// not found.
+	ErrPaymentIDNotFound = errors.New("paymentID not found")
+
+	// ErrPaymentIDAlreadyExists is returned if we try to write a pending
+	// payment whose paymentID already exists.
+	ErrPaymentIDAlreadyExists = errors.New("paymentID already exists")
+)
+
+// PaymentResult wraps a result received from the network after a payment
+// attempt was made.
+type PaymentResult struct {
+	// Preimage is set by the switch in case a sent HTLC was settled.
+	Preimage [32]byte
+
+	// Error is non-nil in case a HTLC send failed, and the HTLC is now
+	// irrevocably cancelled. If the payment failed during forwarding, this
+	// error will be a *ForwardingError.
+	Error error
+}