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/builder.go

@@ -1314,8 +1314,7 @@ func (b *Builder) addEdge(edge *models.ChannelEdgeInfo,
 		default:
 		}
 
-		return NewErrf(ErrNoFundingTransaction, "unable to "+
-			"locate funding tx: %v", err)
+		return fmt.Errorf("%w: %w", ErrNoFundingTransaction, err)
 	}
 
 	// Recreate witness output to be sure that declared in channel edge
@@ -1348,8 +1347,7 @@ func (b *Builder) addEdge(edge *models.ChannelEdgeInfo,
 			return err
 		}
 
-		return NewErrf(ErrInvalidFundingOutput, "output failed "+
-			"validation: %w", err)
+		return fmt.Errorf("%w: %w", ErrInvalidFundingOutput, err)
 	}
 
 	// Now that we have the funding outpoint of the channel, ensure
@@ -1366,8 +1364,8 @@ func (b *Builder) addEdge(edge *models.ChannelEdgeInfo,
 			}
 		}
 
-		return NewErrf(ErrChannelSpent, "unable to fetch utxo for "+
-			"chan_id=%v, chan_point=%v: %v", edge.ChannelID,
+		return fmt.Errorf("%w: unable to fetch utxo for chan_id=%v, "+
+			"chan_point=%v: %w", ErrChannelSpent, scid.ToUint64(),
 			fundingPoint, err)
 	}