Impacted files:
* `barcode_product_query.dart`: removed useless `try` as already `catch`'ed
* `continuous_scan_model.dart`: removed the `codeInvalid` case that could never happen
* `fetched_product.dart`: refactored with explicit constructors and additional exception and connectivity fields; removed the `codeInvalid` case that could never happen
* `new_product_page.dart`: minor refactoring
* `product_dialog_helper.dart`: removed the `codeInvalid` case that could never happen; minor refactoting
* `product_list_item_simple.dart`: removed the `codeInvalid` case that could never happen
* `product_loader_page.dart`: removed useless `try` as already `catch`'ed
* `product_refresher.dart`: added a specific "You're not connected to the internet" error message; refactored using more `FetchedProduct`; removed useless method
* `pubspec.lock`: wtf
* `pubspec.yaml`: added package `connectivity_plus`
* `question_card.dart`: refactored using `FetchedProduct`

Impacted files:
* `fetched_product.dart`: added field `failedPingedHost` where we store the host that we couldn't ping
* `generated_plugin_registrant.cc`: wtf
* `generated_plugins.cmake`: wtf
* `GeneratedPluginRegistrant.swift`: wtf
* `main.dart`: registered `DartPingIOS`
* `product_refresher.dart`: now trying to ping the server if exception and connection
* `pubspec.lock`: wtf
* `pubspec.yaml: added packages `dart_ping` and `dart_ping_ios`

Impacted files:
* `app_en.arb`: 4 new labels when we couldn't retrieve a product (not found, no internet, server down, server error)
* `product_refresher.dart`: used the new labels