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/surfshark/updater/api.go

@@ -3,7 +3,6 @@ package updater
 import (
 	"context"
 	"encoding/json"
-	"errors"
 	"fmt"
 	"net/http"
 
@@ -38,8 +37,6 @@ func addServersFromAPI(ctx context.Context, client *http.Client,
 	return nil
 }
 
-var ErrHTTPStatusCodeNotOK = errors.New("HTTP status code not OK")
-
 type serverData struct {
 	Host     string `json:"connectionName"`
 	Region   string `json:"region"`
@@ -66,7 +63,7 @@ func fetchAPI(ctx context.Context, client *http.Client) (
 		defer response.Body.Close()
 
 		if response.StatusCode != http.StatusOK {
-			return nil, fmt.Errorf("%w: %d %s", ErrHTTPStatusCodeNotOK,
+			return nil, fmt.Errorf("HTTP status code not OK: %d %s",
 				response.StatusCode, response.Status)
 		}
 

internal/provider/surfshark/updater/location.go

@@ -1,20 +1,17 @@
 package updater
 
 import (
-	"errors"
 	"fmt"
 
 	"github.com/qdm12/gluetun/internal/provider/surfshark/servers"
 )
 
-var errHostnameNotFound = errors.New("hostname not found in hostname to location mapping")
-
 func getHostInformation(host string, hostnameToLocation map[string]servers.ServerLocation) (
 	data servers.ServerLocation, err error,
 ) {
 	locationData, ok := hostnameToLocation[host]
 	if !ok {
-		return locationData, fmt.Errorf("%w: %s", errHostnameNotFound, host)
+		return locationData, fmt.Errorf("hostname %s not found in hostname to location mapping", host)
 	}
 
 	return locationData, nil