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/utils/connection_test.go

@@ -19,8 +19,6 @@ import (
 func Test_GetConnection(t *testing.T) {
 	t.Parallel()
 
-	errTest := errors.New("test error")
-
 	testCases := map[string]struct {
 		provider        string
 		filteredServers []models.Server
@@ -30,12 +28,10 @@ func Test_GetConnection(t *testing.T) {
 		ipv6Supported   bool
 		randSource      rand.Source
 		connection      models.Connection
-		errWrapped      error
 		errMessage      string
 	}{
 		"storage filter error": {
-			filterError: errTest,
-			errWrapped:  errTest,
+			filterError: errors.New("test error"),
 			errMessage:  "filtering servers: test error",
 		},
 		"server without IPs": {
@@ -50,7 +46,6 @@ func Test_GetConnection(t *testing.T) {
 				OpenVPNUDPPort: 1,
 				WireguardPort:  1,
 			},
-			errWrapped: ErrNoConnectionToPickFrom,
 			errMessage: "no connection to pick from",
 		},
 		"OpenVPN server with hostname": {
@@ -199,9 +194,10 @@ func Test_GetConnection(t *testing.T) {
 				testCase.randSource)
 
 			assert.Equal(t, testCase.connection, connection)
-			assert.ErrorIs(t, err, testCase.errWrapped)
-			if testCase.errWrapped != nil {
+			if testCase.errMessage != "" {
 				assert.EqualError(t, err, testCase.errMessage)
+			} else {
+				assert.NoError(t, err)
 			}
 		})
 	}

internal/provider/utils/nofetcher.go

@@ -2,7 +2,6 @@ package utils
 
 import (
 	"context"
-	"errors"
 	"fmt"
 
 	"github.com/qdm12/gluetun/internal/models"
@@ -18,10 +17,8 @@ func NewNoFetcher(providerName string) *NoFetcher {
 	}
 }
 
-var ErrFetcherNotSupported = errors.New("fetching of servers is not supported")
-
 func (n *NoFetcher) FetchServers(context.Context, int) (
 	servers []models.Server, err error,
 ) {
-	return nil, fmt.Errorf("%w: for %s", ErrFetcherNotSupported, n.providerName)
+	return nil, fmt.Errorf("fetching of servers is not supported for %s", n.providerName)
 }

internal/provider/utils/pick.go

@@ -11,8 +11,6 @@ import (
 	"github.com/qdm12/gluetun/internal/models"
 )
 
-var ErrNoConnectionToPickFrom = errors.New("no connection to pick from")
-
 // pickConnection picks a connection from a pool of connections.
 // If the VPN protocol is Wireguard and the target IP is set,
 // it finds the connection corresponding to this target IP.
@@ -23,7 +21,7 @@ func pickConnection(connections []models.Connection,
 	connection models.Connection, err error,
 ) {
 	if len(connections) == 0 {
-		return connection, ErrNoConnectionToPickFrom
+		return connection, errors.New("no connection to pick from")
 	}
 
 	var targetIP netip.Addr
@@ -56,8 +54,6 @@ func pickRandomConnection(connections []models.Connection,
 	return connections[rand.New(source).Intn(len(connections))] //nolint:gosec
 }
 
-var errTargetIPNotFound = errors.New("target IP address not found")
-
 func getTargetIPConnection(connections []models.Connection,
 	targetIP netip.Addr,
 ) (connection models.Connection, err error) {
@@ -66,6 +62,6 @@ func getTargetIPConnection(connections []models.Connection,
 			return connection, nil
 		}
 	}
-	return connection, fmt.Errorf("%w: in %d filtered connections",
-		errTargetIPNotFound, len(connections))
+	return connection, fmt.Errorf("target IP address not found: in %d filtered connections",
+		len(connections))
 }