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.

discovery/gossiper.go

@@ -2678,10 +2678,9 @@ func (d *AuthenticatedGossiper) handleChanAnnouncement(nMsg *networkMsg,
 
 			return anns, true
 
-		case graph.IsError(
-			err, graph.ErrNoFundingTransaction,
-			graph.ErrInvalidFundingOutput,
-		):
+		case errors.Is(err, graph.ErrNoFundingTransaction),
+			errors.Is(err, graph.ErrInvalidFundingOutput):
+
 			key := newRejectCacheKey(
 				scid.ToUint64(),
 				sourceToPub(nMsg.source),
@@ -2695,7 +2694,7 @@ func (d *AuthenticatedGossiper) handleChanAnnouncement(nMsg *networkMsg,
 				d.banman.incrementBanScore(nMsg.peer.PubKey())
 			}
 
-		case graph.IsError(err, graph.ErrChannelSpent):
+		case errors.Is(err, graph.ErrChannelSpent):
 			key := newRejectCacheKey(
 				scid.ToUint64(),
 				sourceToPub(nMsg.source),