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/windscribe/connection_test.go

@@ -22,21 +22,17 @@ func Test_Provider_GetConnection(t *testing.T) {
 
 	const provider = providers.Windscribe
 
-	errTest := errors.New("test error")
-
 	testCases := map[string]struct {
 		filteredServers []models.Server
 		storageErr      error
 		selection       settings.ServerSelection
 		ipv6Supported   bool
 		connection      models.Connection
-		errWrapped      error
 		errMessage      string
 		panicMessage    string
 	}{
 		"error": {
-			storageErr: errTest,
-			errWrapped: errTest,
+			storageErr: errors.New("test error"),
 			errMessage: "filtering servers: test error",
 		},
 		"default OpenVPN TCP port": {
@@ -111,9 +107,10 @@ func Test_Provider_GetConnection(t *testing.T) {
 
 			connection, err := provider.GetConnection(testCase.selection, testCase.ipv6Supported)
 
-			assert.ErrorIs(t, err, testCase.errWrapped)
-			if testCase.errWrapped != nil {
+			if testCase.errMessage != "" {
 				assert.EqualError(t, err, testCase.errMessage)
+			} else {
+				assert.NoError(t, err)
 			}
 
 			assert.Equal(t, testCase.connection, connection)

internal/provider/windscribe/updater/api.go

@@ -3,7 +3,6 @@ package updater
 import (
 	"context"
 	"encoding/json"
-	"errors"
 	"fmt"
 	"net/http"
 	"net/netip"
@@ -11,8 +10,6 @@ import (
 	"time"
 )
 
-var ErrHTTPStatusCodeNotOK = errors.New("HTTP status code not OK")
-
 type apiData struct {
 	Data []regionData `json:"data"`
 }
@@ -55,7 +52,7 @@ func fetchAPI(ctx context.Context, client *http.Client) (
 	defer response.Body.Close()
 
 	if response.StatusCode != http.StatusOK {
-		return data, fmt.Errorf("%w: %d %s", ErrHTTPStatusCodeNotOK,
+		return data, fmt.Errorf("HTTP status code not OK: %d %s",
 			response.StatusCode, response.Status)
 	}
 

internal/provider/windscribe/updater/servers.go

@@ -2,7 +2,6 @@ package updater
 
 import (
 	"context"
-	"errors"
 	"fmt"
 	"net/netip"
 	"sort"
@@ -12,8 +11,6 @@ import (
 	"github.com/qdm12/gluetun/internal/provider/common"
 )
 
-var ErrNoWireguardKey = errors.New("no wireguard public key found")
-
 func (u *Updater) FetchServers(ctx context.Context, minServers int) (
 	servers []models.Server, err error,
 ) {
@@ -51,7 +48,7 @@ func (u *Updater) FetchServers(ctx context.Context, minServers int) (
 				if !node.IP3.IsValid() { // Wireguard + Stealth
 					continue
 				} else if wgPubKey == "" {
-					return nil, fmt.Errorf("%w: for node %s", ErrNoWireguardKey, node.Hostname)
+					return nil, fmt.Errorf("no wireguard public key found: for node %s", node.Hostname)
 				}
 
 				server.VPN = vpn.Wireguard