routing: add new error for invalid funding tx rejection

routing/errors.go

@@ -20,6 +20,10 @@ const (
 	// the purported funding output has actually already been spent on
 	// chain.
 	ErrChannelSpent
+
+	// ErrNoFundingTransaction is returned when we are unable to find the
+	// funding transaction described by the short channel ID on chain.
+	ErrNoFundingTransaction
 )
 
 // routerError is a structure that represent the error inside the routing package,