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/custom/connection.go

@@ -1,7 +1,6 @@
 package custom
 
 import (
-	"errors"
 	"fmt"
 
 	"github.com/qdm12/gluetun/internal/configuration/settings"
@@ -10,8 +9,6 @@ import (
 	"github.com/qdm12/gluetun/internal/models"
 )
 
-var ErrVPNTypeNotSupported = errors.New("VPN type not supported for custom provider")
-
 // GetConnection gets the connection from the OpenVPN configuration file.
 func (p *Provider) GetConnection(selection settings.ServerSelection, _ bool) (
 	connection models.Connection, err error,
@@ -22,7 +19,7 @@ func (p *Provider) GetConnection(selection settings.ServerSelection, _ bool) (
 	case vpn.Wireguard, vpn.AmneziaWg:
 		return getWireguardConnection(selection), nil
 	default:
-		return connection, fmt.Errorf("%w: %s", ErrVPNTypeNotSupported, selection.VPN)
+		return connection, fmt.Errorf("VPN type not supported for custom provider: %s", selection.VPN)
 	}
 }
 

internal/provider/custom/openvpnconf.go

@@ -1,7 +1,6 @@
 package custom
 
 import (
-	"errors"
 	"fmt"
 	"strconv"
 	"strings"
@@ -12,8 +11,6 @@ import (
 	"github.com/qdm12/gluetun/internal/provider/utils"
 )
 
-var ErrExtractData = errors.New("failed extracting information from custom configuration file")
-
 func (p *Provider) OpenVPNConfig(connection models.Connection,
 	settings settings.OpenVPN, ipv6Supported bool,
 ) (lines []string) {