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/fastestvpn/updater/api.go

@@ -3,14 +3,11 @@ package updater
 import (
 	"bytes"
 	"context"
-	"errors"
 	"fmt"
 	"io"
 	"net/http"
 	"net/url"
 	"strings"
-
-	"github.com/qdm12/gluetun/internal/provider/common"
 )
 
 type apiServer struct {
@@ -19,8 +16,6 @@ type apiServer struct {
 	hostname string
 }
 
-var ErrDataMalformed = errors.New("data is malformed")
-
 const apiURL = "https://support.fastestvpn.com/wp-admin/admin-ajax.php"
 
 // The API URL and requests are shamelessly taken from network operations
@@ -49,7 +44,7 @@ func fetchAPIServers(ctx context.Context, client *http.Client, protocol string)
 
 	if response.StatusCode != http.StatusOK {
 		_ = response.Body.Close()
-		return nil, fmt.Errorf("%w: %d", common.ErrHTTPStatusCodeNotOK, response.StatusCode)
+		return nil, fmt.Errorf("HTTP status code not OK: %d", response.StatusCode)
 	}
 
 	data, err := io.ReadAll(response.Body)
@@ -79,8 +74,8 @@ func fetchAPIServers(ctx context.Context, client *http.Client, protocol string)
 		for i := range numberOfTDBlocks {
 			tdBlock := getNextTDBlock(trBlock)
 			if tdBlock == nil {
-				return nil, fmt.Errorf("%w: expected 3 <td> blocks in <tr> block %q",
-					ErrDataMalformed, string(trBlock))
+				return nil, fmt.Errorf("data is malformed: expected 3 <td> blocks in <tr> block %q",
+					string(trBlock))
 			}
 			trBlock = trBlock[len(tdBlock):]