Fix missing locale errors in the options page test enviroment

[saroyanm]

Environment

All browsers

How to reproduce

1. Clone ​adblockplusui repository
2. Run test enviroment
3. Open desktop-options.html
4. Observe

Observed behaviour

The strings are not loaded and current errors are thrown:

common.js:146 GET http://127.0.0.1:5000/locale/en/common.json 404 (File not found)
common.js:146 GET http://127.0.0.1:5000/locale/en/desktop-options.json 404 (File not found)

Expected behaviour

• Strings are loaded, no errors are thrown
• Ideally we would like to also throw error when requested string is not available (ex. Unknown localization message $msgId$).

[saroyanm]

The issue here seems to be ​xhr error handling, while it seems like error handling here is tricky maybe we can use fetch API.

I haven't make it part of #5860 intentionally as the priority of this issue is lower than the other one.

[greiner]

Closing this ticket because we have since added translation file linting to ensure those files are valid (see ​ui#82). Furthermore, any errors relating to missing translation files are either expected (i.e. we don't have translations for a given file yet) or an actual error (e.g. we forgot to remove the reference to a translation file that no longer exists).