chore: do not use sentinel errors when unneeded

- main reason being it's a burden to always define sentinel errors at global scope, wrap them with `%w` instead of using a string directly
- only use sentinel errors when it has to be checked using `errors.Is`
- replace all usage of these sentinel errors in `fmt.Errorf` with direct strings that were in the sentinel error
- exclude the sentinel error definition requirement from .golangci.yml
- update unit tests to use ContainersError instead of ErrorIs so it stays as a "not a change detector test" without requiring a sentinel error

internal/provider/vpnsecure/updater/website.go

@@ -33,8 +33,6 @@ func fetchServers(ctx context.Context, client *http.Client,
 	return servers, nil
 }
 
-var ErrHTMLServersDivNotFound = errors.New("HTML servers container div not found")
-
 const divString = "div"
 
 func parseHTML(rootNode *html.Node) (servers []models.Server,
@@ -43,7 +41,7 @@ func parseHTML(rootNode *html.Node) (servers []models.Server,
 	// Find div container for all servers, searching with BFS.
 	serversDiv := findServersDiv(rootNode)
 	if serversDiv == nil {
-		return nil, nil, htmlutils.WrapError(ErrHTMLServersDivNotFound, rootNode)
+		return nil, nil, htmlutils.WrapError(errors.New("HTML servers container div not found"), rootNode)
 	}
 
 	for countryNode := serversDiv.FirstChild; countryNode != nil; countryNode = countryNode.NextSibling {