lntest/harness: improve error message for AssertChannelExists

predErr wasn't always being set when the predicate failed, replace with
wait.NoError.

lntest/harness.go

@@ -1195,28 +1195,25 @@ func (n *NetworkHarness) AssertChannelExists(ctx context.Context,
 
 	req := &lnrpc.ListChannelsRequest{}
 
-	var predErr error
-	pred := func() bool {
+	return wait.NoError(func() error {
 		resp, err := node.ListChannels(ctx, req)
 		if err != nil {
-			predErr = fmt.Errorf("unable fetch node's channels: %v", err)
-			return false
+			return fmt.Errorf("unable fetch node's channels: %v", err)
 		}
 
 		for _, channel := range resp.Channels {
 			if channel.ChannelPoint == chanPoint.String() {
-				return channel.Active
+				if channel.Active {
+					return nil
+				}
+
+				return fmt.Errorf("channel %s inactive",
+					chanPoint)
 			}
-
 		}
-		return false
-	}
 
-	if err := wait.Predicate(pred, time.Second*15); err != nil {
-		return fmt.Errorf("channel not found: %v", predErr)
-	}
-
-	return nil
+		return fmt.Errorf("channel %s not found", chanPoint)
+	}, 15*time.Second)
 }
 
 // DumpLogs reads the current logs generated by the passed node, and returns