fix: folksonomy getProductTag may return 404

[monsieurtanuki]

What

• Recent change in folksonomy, server-side (couple of days ago): unknown product tag search will return 404.
• This PR deals with it.

❌ test/api_folksonomy_test.dart: OpenFoodAPIClient Folksonomy getProductTag - not found (failed)
  Exception: Wrong status code: 404
  package:openfoodfacts/src/folksonomy.dart 373:7  FolksonomyAPIClient._checkResponse
  package:openfoodfacts/src/folksonomy.dart 172:5  FolksonomyAPIClient.getProductTag
  ===== asynchronous gap ===========================
  test/api_folksonomy_test.dart 134:36             main.<fn>.<fn>
```

[monsieurtanuki]

Thank you @teolemon for your review!