Fix and improve French translations

[deronnax]

Follow-up of #9770 where I offered to do a re-review of the French translation.

1 fix:

• Update french translation #9770 introduced a misspelling: "charactère". This word doesn't exist in French, it looks like a mash-up of the French and English term

3 improvments:

• « Pas trouvé » (clumsy, famililiar, non natural) -> « Non trouvé »
• "Entrée invalide" -> "Saisie invalide" (more formal, less ambiguous)
• « Verifer que » -> « Assurez-vous que » (align with the rest of the file)

[browniebroke]

Merci!