chore(all): review error wrappings

- remove repetitive `cannot` and `failed` prefixes
- rename `unmarshaling` to `decoding`

internal/provider/mullvad/connection_test.go

@@ -37,7 +37,7 @@ func Test_Provider_GetConnection(t *testing.T) {
 		"error": {
 			storageErr: errTest,
 			errWrapped: errTest,
-			errMessage: "cannot filter servers: test error",
+			errMessage: "filtering servers: test error",
 		},
 		"default OpenVPN TCP port": {
 			filteredServers: []models.Server{

internal/provider/mullvad/updater/api.go

@@ -9,8 +9,8 @@ import (
 )
 
 var (
-	ErrHTTPStatusCodeNotOK   = errors.New("HTTP status code not OK")
-	ErrUnmarshalResponseBody = errors.New("failed unmarshaling response body")
+	ErrHTTPStatusCodeNotOK = errors.New("HTTP status code not OK")
+	ErrDecodeResponseBody  = errors.New("failed decoding response body")
 )
 
 type serverData struct {
@@ -47,7 +47,7 @@ func fetchAPI(ctx context.Context, client *http.Client) (data []serverData, err
 
 	decoder := json.NewDecoder(response.Body)
 	if err := decoder.Decode(&data); err != nil {
-		return nil, fmt.Errorf("%w: %s", ErrUnmarshalResponseBody, err)
+		return nil, fmt.Errorf("%w: %s", ErrDecodeResponseBody, err)
 	}
 
 	if err := response.Body.Close(); err != nil {