htlcswitch: introduce wrapper type error encrypter to identify blinded

Introduce two wrapper types for our existing SphinxErrorEncrypter
that are used to represent error encrypters where we're a part of a
blinded route. These encrypters are functionally the same as a sphinx
encrypter, and are just used as "markers" so that we know that we
need to handle our error differently due to our different role.

We need to persist this information to account for restart cases where
we've resovled the outgoing HTLC, then restart and need to handle the
error for the incoming link. Specifically, this is relevant for:
- On chain resolution messages received after restart
- Forwarding packages that are re-forwarded after restart

This is also generally helpful, because we can store this information
in one place (the circuit) rather than trying to reconstruct it in
various places when forwarding the failure back over the switch.

htlcswitch/circuit.go

@@ -205,6 +205,12 @@ func (c *PaymentCircuit) Decode(r io.Reader) error {
 		// Test encrypter.
 		c.ErrorEncrypter = NewMockObfuscator()
 
+	case hop.EncrypterTypeIntroduction:
+		c.ErrorEncrypter = hop.NewIntroductionErrorEncrypter()
+
+	case hop.EncrypterTypeRelaying:
+		c.ErrorEncrypter = hop.NewRelayingErrorEncrypter()
+
 	default:
 		return UnknownEncrypterType(encrypterType)
 	}