Fix spelling of two French user messages

Context

Two French messages shown to users contain a spelling mistake:

• frontend/app/adapters/application.js, message displayed on a server error: "Veuillez réesayer ultérieurement" instead of "réessayer".
• pkg/handlers/results.go, NotFound: "La resource demandée" instead of "ressource".

Task

• Fix both messages.

Acceptance criteria

• Both messages are spelled correctly; the NotFound message is covered by a test.