multi: fix fmt.Errorf error wrapping

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

lntest/node/watcher.go

@@ -245,7 +245,7 @@ func (nw *nodeWatcher) WaitForChannelPolicyUpdate(
 		case <-timer:
 			expected, err := json.MarshalIndent(policy, "", "\t")
 			if err != nil {
-				return fmt.Errorf("encode policy err: %v", err)
+				return fmt.Errorf("encode policy err: %w", err)
 			}
 			policies, err := syncMapToJSON(
 				&nw.state.policyUpdates.Map,
@@ -274,7 +274,7 @@ func syncMapToJSON(state *sync.Map) ([]byte, error) {
 	})
 	policies, err := json.MarshalIndent(m, "", "\t")
 	if err != nil {
-		return nil, fmt.Errorf("encode polices err: %v", err)
+		return nil, fmt.Errorf("encode polices err: %w", err)
 	}
 
 	return policies, nil