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/mullvad/connection_test.go

@@ -22,20 +22,16 @@ func Test_Provider_GetConnection(t *testing.T) {
 
 	const provider = providers.Mullvad
 
-	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 Wireguard port": {
@@ -70,9 +66,10 @@ 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/mullvad/updater/api.go

@@ -3,16 +3,10 @@ package updater
 import (
 	"context"
 	"encoding/json"
-	"errors"
 	"fmt"
 	"net/http"
 )
 
-var (
-	ErrHTTPStatusCodeNotOK = errors.New("HTTP status code not OK")
-	ErrDecodeResponseBody  = errors.New("failed decoding response body")
-)
-
 type serverData struct {
 	Hostname string `json:"hostname"`
 	Country  string `json:"country_name"`
@@ -41,13 +35,12 @@ func fetchAPI(ctx context.Context, client *http.Client) (data []serverData, err
 	defer response.Body.Close()
 
 	if response.StatusCode != http.StatusOK {
-		return nil, fmt.Errorf("%w: %d %s", ErrHTTPStatusCodeNotOK,
-			response.StatusCode, response.Status)
+		return nil, fmt.Errorf("HTTP status code not OK: %d %s", response.StatusCode, response.Status)
 	}
 
 	decoder := json.NewDecoder(response.Body)
 	if err := decoder.Decode(&data); err != nil {
-		return nil, fmt.Errorf("%w: %s", ErrDecodeResponseBody, err)
+		return nil, fmt.Errorf("failed decoding response body: %s", err)
 	}
 
 	if err := response.Body.Close(); err != nil {

internal/provider/mullvad/updater/hosttoserver.go

@@ -12,20 +12,13 @@ import (
 
 type hostToServer map[string]models.Server
 
-var (
-	ErrNoIP                = errors.New("no IP address for VPN server")
-	ErrIPIsNotV4           = errors.New("IP address is not IPv4")
-	ErrIPIsNotV6           = errors.New("IP address is not IPv6")
-	ErrVPNTypeNotSupported = errors.New("VPN type not supported")
-)
-
 func (hts hostToServer) add(data serverData) (err error) {
 	if !data.Active {
 		return nil
 	}
 
 	if data.IPv4 == "" && data.IPv6 == "" {
-		return fmt.Errorf("%w", ErrNoIP)
+		return errors.New("no IP address for VPN server")
 	}
 
 	server, ok := hts[data.Hostname]
@@ -40,7 +33,7 @@ func (hts hostToServer) add(data serverData) (err error) {
 		// ignore bridge servers
 		return nil
 	default:
-		return fmt.Errorf("%w: %s", ErrVPNTypeNotSupported, data.Type)
+		return fmt.Errorf("VPN type not supported: %s", data.Type)
 	}
 
 	if data.IPv4 != "" {
@@ -48,7 +41,7 @@ func (hts hostToServer) add(data serverData) (err error) {
 		if err != nil {
 			return fmt.Errorf("parsing IPv4 address: %w", err)
 		} else if !ipv4.Is4() {
-			return fmt.Errorf("%w: %s", ErrIPIsNotV4, data.IPv4)
+			return fmt.Errorf("IP address is not IPv4: %s", data.IPv4)
 		}
 		server.IPs = append(server.IPs, ipv4)
 	}
@@ -58,7 +51,7 @@ func (hts hostToServer) add(data serverData) (err error) {
 		if err != nil {
 			return fmt.Errorf("parsing IPv6 address: %w", err)
 		} else if !ipv6.Is6() {
-			return fmt.Errorf("%w: %s", ErrIPIsNotV6, data.IPv6)
+			return fmt.Errorf("IP address is not IPv6: %s", data.IPv6)
 		}
 		server.IPs = append(server.IPs, ipv6)
 	}