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/privatevpn/portforward.go

@@ -11,14 +11,11 @@ import (
 	"strconv"
 	"time"
 
-	"github.com/qdm12/gluetun/internal/provider/common"
 	"github.com/qdm12/gluetun/internal/provider/utils"
 )
 
 var regexPort = regexp.MustCompile(`[1-9][0-9]{0,4}`)
 
-var ErrPortForwardedNotFound = errors.New("port forwarded not found")
-
 // PortForward obtains a VPN server side port forwarded from the PrivateVPN API.
 // It returns 0 if all ports are to forwarded on a dedicated server IP.
 func (p *Provider) PortForward(ctx context.Context, objects utils.PortForwardObjects) (
@@ -42,8 +39,7 @@ func (p *Provider) PortForward(ctx context.Context, objects utils.PortForwardObj
 	}
 
 	if response.StatusCode != http.StatusOK {
-		return nil, fmt.Errorf("%w: %d %s", common.ErrHTTPStatusCodeNotOK,
-			response.StatusCode, response.Status)
+		return nil, fmt.Errorf("HTTP status code not OK: %d %s", response.StatusCode, response.Status)
 	}
 
 	defer response.Body.Close()
@@ -62,12 +58,12 @@ func (p *Provider) PortForward(ctx context.Context, objects utils.PortForwardObj
 		return nil, fmt.Errorf("decoding JSON response: %w; data is: %s",
 			err, string(bytes))
 	} else if !data.Supported {
-		return nil, fmt.Errorf("%w for this VPN server", common.ErrPortForwardNotSupported)
+		return nil, errors.New("port forwarding not supported for this VPN server")
 	}
 
 	portString := regexPort.FindString(data.Status)
 	if portString == "" {
-		return nil, fmt.Errorf("%w: in status %q", ErrPortForwardedNotFound, data.Status)
+		return nil, fmt.Errorf("port forwarded not found in status %q", data.Status)
 	}
 
 	const base, bitSize = 10, 16

internal/provider/privatevpn/portforward_test.go

@@ -22,8 +22,6 @@ func (s roundTripFunc) RoundTrip(r *http.Request) (*http.Response, error) {
 func Test_Provider_PortForward(t *testing.T) {
 	t.Parallel()
 
-	errTest := errors.New("test error")
-
 	canceledCtx, cancel := context.WithCancel(context.Background())
 	cancel()
 
@@ -59,7 +57,7 @@ func Test_Provider_PortForward(t *testing.T) {
 						assert.Equal(t,
 							"https://connect.pvdatanet.com/v3/Api/port?ip[]=10.10.10.10",
 							r.URL.String())
-						return nil, errTest
+						return nil, errors.New("test error")
 					}),
 				},
 			},
@@ -156,7 +154,7 @@ func Test_Provider_PortForward(t *testing.T) {
 					}),
 				},
 			},
-			errMessage: "port forwarded not found: in status \"no port here\"",
+			errMessage: "port forwarded not found in status \"no port here\"",
 		},
 		"port_too_big": {
 			ctx: context.Background(),

internal/provider/privatevpn/updater/filename.go

@@ -1,7 +1,6 @@
 package updater
 
 import (
-	"errors"
 	"fmt"
 	"regexp"
 	"strings"
@@ -9,12 +8,6 @@ import (
 
 var trailingNumber = regexp.MustCompile(` [0-9]+$`)
 
-var (
-	errBadPrefix      = errors.New("bad prefix in file name")
-	errBadSuffix      = errors.New("bad suffix in file name")
-	errNotEnoughParts = errors.New("not enough parts in file name")
-)
-
 func parseFilename(fileName string) (
 	countryCode, city string, err error,
 ) {
@@ -22,7 +15,7 @@ func parseFilename(fileName string) (
 
 	const prefix = "PrivateVPN-"
 	if !strings.HasPrefix(fileName, prefix) {
-		return "", "", fmt.Errorf("%w: %s", errBadPrefix, fileName)
+		return "", "", fmt.Errorf("bad prefix in file name %s", fileName)
 	}
 	s := strings.TrimPrefix(fileName, prefix)
 
@@ -34,7 +27,7 @@ func parseFilename(fileName string) (
 	case strings.HasSuffix(fileName, udpSuffix):
 		s = strings.TrimSuffix(s, udpSuffix)
 	default:
-		return "", "", fmt.Errorf("%w: %s", errBadSuffix, fileName)
+		return "", "", fmt.Errorf("bad suffix in file name %s", fileName)
 	}
 
 	s = trailingNumber.ReplaceAllString(s, "")
@@ -42,7 +35,7 @@ func parseFilename(fileName string) (
 	parts := strings.Split(s, "-")
 	const minParts = 2
 	if len(parts) < minParts {
-		return "", "", fmt.Errorf("%w: %s", errNotEnoughParts, fileName)
+		return "", "", fmt.Errorf("not enough parts in file name %s", fileName)
 	}
 	countryCode, city = parts[0], parts[1]