discovery+graph: convert errors from codes to variables

In preparation for moving funding transaction validiation from the
Builder to the Gossiper in later commit, we first convert these graph
Error Codes to normal error variables. This will help make the later
commit a pure code move.

graph/errors.go

@@ -2,6 +2,25 @@ package graph
 
 import "github.com/go-errors/errors"
 
+var (
+	// ErrNoFundingTransaction is returned when we are unable to find the
+	// funding transaction described by the short channel ID on chain.
+	ErrNoFundingTransaction = errors.New(
+		"unable to find the funding transaction",
+	)
+
+	// ErrInvalidFundingOutput is returned if the channel funding output
+	// fails validation.
+	ErrInvalidFundingOutput = errors.New(
+		"channel funding output validation failed",
+	)
+
+	// ErrChannelSpent is returned when we go to validate a channel, but
+	// the purported funding output has actually already been spent on
+	// chain.
+	ErrChannelSpent = errors.New("channel output has been spent")
+)
+
 // ErrorCode is used to represent the various errors that can occur within this
 // package.
 type ErrorCode uint8
@@ -15,19 +34,6 @@ const (
 	// this update can't bring us something new, or because a node
 	// announcement was given for node not found in any channel.
 	ErrIgnored
-
-	// ErrChannelSpent is returned when we go to validate a channel, but
-	// 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
-
-	// ErrInvalidFundingOutput is returned if the channel funding output
-	// fails validation.
-	ErrInvalidFundingOutput
 )
 
 // Error is a structure that represent the error inside the graph package,