multi: fix fmt.Errorf error wrapping

Refactor fmt.Errorf usage to correctly wrap errors instead of using
non-wrapping format verbs.

contractcourt/breach_arbitrator.go

@@ -924,7 +924,7 @@ func (b *BreachArbitrator) cleanupBreach(chanPoint *wire.OutPoint) error {
 	// With the channel closed, mark it in the database as such.
 	err := b.cfg.DB.MarkChanFullyClosed(chanPoint)
 	if err != nil {
-		return fmt.Errorf("unable to mark chan as closed: %v", err)
+		return fmt.Errorf("unable to mark chan as closed: %w", err)
 	}
 
 	// Justice has been carried out; we can safely delete the retribution

contractcourt/chain_watcher.go

@@ -1202,7 +1202,7 @@ func (c *chainWatcher) dispatchContractBreach(spendEvent *chainntnfs.SpendDetail
 		c.cfg.chanState.FundingOutpoint, broadcastStateNum)
 
 	if err := c.cfg.chanState.MarkBorked(); err != nil {
-		return fmt.Errorf("unable to mark channel as borked: %v", err)
+		return fmt.Errorf("unable to mark channel as borked: %w", err)
 	}
 
 	spendHeight := uint32(spendEvent.SpendingHeight)