Remove 'if err == nil' anti-pattern

Using 'if err == nil' is an anti-pattern in Go and should be avoided
since most if not all Go developers expect 'if err != nil {...}'

This patch flips/removes the checks in the main client code but leaves
the checks in the test cases were we require an error.

client.go

@@ -1030,24 +1030,25 @@ func (c *Client) ReadWithContext(ctx context.Context, req *ua.ReadRequest) (*ua.
 	err := c.SendWithContext(ctx, req, func(v interface{}) error {
 		err := safeAssign(v, &res)
 
+		if err != nil {
+			return err
+		}
+
 		// If the client cannot decode an extension object then its
 		// value will be nil. However, since the EO was known to the
 		// server the StatusCode for that data value will be OK. We
 		// therefore check for extension objects with nil values and set
 		// the status code to StatusBadDataTypeIDUnknown.
-		if err == nil {
-			for _, dv := range res.Results {
-				if dv.Value == nil {
-					continue
-				}
-				val := dv.Value.Value()
-				if eo, ok := val.(*ua.ExtensionObject); ok && eo.Value == nil {
-					dv.Status = ua.StatusBadDataTypeIDUnknown
-				}
+		for _, dv := range res.Results {
+			if dv.Value == nil {
+				continue
+			}
+			val := dv.Value.Value()
+			if eo, ok := val.(*ua.ExtensionObject); ok && eo.Value == nil {
+				dv.Status = ua.StatusBadDataTypeIDUnknown
 			}
 		}
-
-		return err
+		return nil
 	})
 	return res, err
 }

stats/stats.go

@@ -43,11 +43,10 @@ func (s *Stats) Reset() {
 
 // RecordError updates the metric for an error by one.
 func (s *Stats) RecordError(err error) {
-	if err == nil {
-		return
-	}
 	var code ua.StatusCode
 	switch {
+	case err == nil:
+		return
 	case errors.Is(err, io.EOF):
 		s.Error.Add("io.EOF", 1)
 	case errors.Is(err, ua.StatusOK):

subscription.go

@@ -173,17 +173,18 @@ func (s *Subscription) UnmonitorWithContext(ctx context.Context, monitoredItemID
 	err := s.c.SendWithContext(ctx, req, func(v interface{}) error {
 		return safeAssign(v, &res)
 	})
+	if err != nil {
+		return nil, err
+	}
 
-	if err == nil {
-		// remove monitored items
-		s.itemsMu.Lock()
-		for _, id := range monitoredItemIDs {
-			delete(s.items, id)
-		}
-		s.itemsMu.Unlock()
+	// remove monitored items
+	s.itemsMu.Lock()
+	for _, id := range monitoredItemIDs {
+		delete(s.items, id)
 	}
+	s.itemsMu.Unlock()
 
-	return res, err
+	return res, nil
 }
 
 // Note: Starting with v0.5 this method will require a context