htlcswitch: use LinkError for internal errors

Update the ChannelLink interface to specifically
return the LinkError struct. This error implements
the ClearTextError interface, so will be picked
up as a routing realted error by the router.

With LinkErrors implemented, the switch now
returns a LinkError for all failures on our
incoming/outgoing link and ForwardingError when
the failure occurs down the line.

htlcswitch/failure_detail.go

@@ -0,0 +1,58 @@
+package htlcswitch
+
+// FailureDetail is an enum which is used to enrich failures with
+// additional information.
+type FailureDetail int
+
+const (
+	// FailureDetailNone is returned when the wire message contains
+	// sufficient information.
+	FailureDetailNone = iota
+
+	// FailureDetailOnionDecode indicates that we could not decode an
+	// onion error.
+	FailureDetailOnionDecode
+
+	// FailureDetailLinkNotEligible indicates that a routing attempt was
+	// made over a link that is not eligible for routing.
+	FailureDetailLinkNotEligible
+
+	// FailureDetailOnChainTimeout indicates that a payment had to be timed
+	// out on chain before it got past the first hop by us or the remote
+	// party.
+	FailureDetailOnChainTimeout
+
+	// FailureDetailHTLCExceedsMax is returned when a htlc exceeds our
+	// policy's maximum htlc amount.
+	FailureDetailHTLCExceedsMax
+
+	// FailureDetailInsufficientBalance is returned when we cannot route a
+	// htlc due to insufficient outgoing capacity.
+	FailureDetailInsufficientBalance
+)
+
+// String returns the string representation of a failure detail.
+func (fd FailureDetail) String() string {
+	switch fd {
+	case FailureDetailNone:
+		return "no failure detail"
+
+	case FailureDetailOnionDecode:
+		return "could not decode onion"
+
+	case FailureDetailLinkNotEligible:
+		return "link not eligible"
+
+	case FailureDetailOnChainTimeout:
+		return "payment was resolved on-chain, then canceled back"
+
+	case FailureDetailHTLCExceedsMax:
+		return "htlc exceeds maximum policy amount"
+
+	case FailureDetailInsufficientBalance:
+		return "insufficient bandwidth to route htlc"
+
+	default:
+		return "unknown failure detail"
+	}
+}