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/ivpn/connection_test.go

@@ -22,20 +22,16 @@ func Test_Provider_GetConnection(t *testing.T) {
 
 	const provider = providers.Ivpn
 
-	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
 	}{
 		"error": {
-			storageErr: errTest,
-			errWrapped: errTest,
+			storageErr: errors.New("test error"),
 			errMessage: "filtering servers: test error",
 		},
 		"default OpenVPN TCP port": {
@@ -104,9 +100,10 @@ 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/ivpn/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")
-
 type apiData struct {
 	Servers []apiServer `json:"servers"`
 }
@@ -45,8 +42,8 @@ func fetchAPI(ctx context.Context, client *http.Client) (
 
 	if response.StatusCode != http.StatusOK {
 		_ = response.Body.Close()
-		return data, fmt.Errorf("%w: %d %s",
-			errHTTPStatusCodeNotOK, response.StatusCode, response.Status)
+		return data, fmt.Errorf("HTTP status code not OK: %d %s",
+			response.StatusCode, response.Status)
 	}
 
 	decoder := json.NewDecoder(response.Body)