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/nordvpn/updater/api.go

@@ -3,13 +3,10 @@ package updater
 import (
 	"context"
 	"encoding/json"
-	"errors"
 	"fmt"
 	"net/http"
 )
 
-var ErrHTTPStatusCodeNotOK = errors.New("HTTP status code not OK")
-
 func fetchAPI(ctx context.Context, client *http.Client,
 	limit uint,
 ) (data serversData, err error) {
@@ -28,7 +25,7 @@ func fetchAPI(ctx context.Context, client *http.Client,
 	defer response.Body.Close()
 
 	if response.StatusCode != http.StatusOK {
-		return serversData{}, fmt.Errorf("%w: %s", ErrHTTPStatusCodeNotOK, response.Status)
+		return serversData{}, fmt.Errorf("HTTP status code not OK: %s", response.Status)
 	}
 
 	decoder := json.NewDecoder(response.Body)

internal/provider/nordvpn/updater/models.go

@@ -168,11 +168,6 @@ func (s *serverData) ips() (ips []netip.Addr) {
 	return ips
 }
 
-var (
-	ErrWireguardPublicKeyMalformed = errors.New("wireguard public key is malformed")
-	ErrWireguardPublicKeyNotFound  = errors.New("wireguard public key not found")
-)
-
 // wireguardPublicKey returns the Wireguard public key for the server.
 func (s *serverData) wireguardPublicKey(technologies map[uint32]technologyData) (
 	wgPubKey string, err error,
@@ -189,11 +184,10 @@ func (s *serverData) wireguardPublicKey(technologies map[uint32]technologyData)
 			wgPubKey = metadata.Value
 			_, err = base64.StdEncoding.DecodeString(wgPubKey)
 			if err != nil {
-				return "", fmt.Errorf("%w: %s cannot be decoded: %s",
-					ErrWireguardPublicKeyMalformed, wgPubKey, err)
+				return "", fmt.Errorf("wireguard public key is malformed: %s cannot be decoded: %s", wgPubKey, err)
 			}
 			return metadata.Value, nil
 		}
 	}
-	return "", fmt.Errorf("%w", ErrWireguardPublicKeyNotFound)
+	return "", errors.New("wireguard public key not found")
 }

internal/provider/nordvpn/updater/name.go

@@ -7,10 +7,7 @@ import (
 	"strings"
 )
 
-var (
-	ErrNoIDInServerName      = errors.New("no ID in server name")
-	ErrInvalidIDInServerName = errors.New("invalid ID in server name")
-)
+var ErrNoIDInServerName = errors.New("no ID in server name")
 
 func parseServerName(serverName string) (number uint16, err error) {
 	i := strings.IndexRune(serverName, '#')
@@ -21,7 +18,7 @@ func parseServerName(serverName string) (number uint16, err error) {
 	idString := serverName[i+1:]
 	idUint64, err := strconv.ParseUint(idString, 10, 16)
 	if err != nil {
-		return 0, fmt.Errorf("%w: %s", ErrInvalidIDInServerName, serverName)
+		return 0, fmt.Errorf("invalid ID in server name: %s", serverName)
 	}
 
 	number = uint16(idUint64)

internal/provider/nordvpn/updater/servers.go

@@ -11,8 +11,6 @@ import (
 	"github.com/qdm12/gluetun/internal/provider/common"
 )
 
-var ErrNotIPv4 = errors.New("IP address is not IPv4")
-
 func (u *Updater) FetchServers(ctx context.Context, minServers int) (
 	servers []models.Server, err error,
 ) {