fix: reset password checks with languages

[monsieurtanuki]

What

• Not that long ago there was no country or language parameters on reset password. We lousily tried to detect sentences in the resulting html.
• Now that we have country or language parameters, we get a localized html. The sentences we expected are localized. That's why in this PR we use html tags instead of localized terms.
• That's still very lousy: we would be better off calling a more appropriate server URL that would return a json. But that does not exist AFAIK.

Part of

• Still getting errors when attempting to reset my password smooth-app#4549

Impacted files

• open_food_api_client.dart: relying on html tags for error detection instead of English localizations
• user_management_test_prod.dart: additional tests with / without language/country and on existing / not existing users

[g123k]

A better implementation indeed.
FYI, I know there is an opened issue to have error codes instead, but during the meantime, it's still better

[monsieurtanuki]

Thank you @g123k for your review!