fix(unremind): the command does not indicate that a reminder does not exist

[Akinator31]

This pull request introduces a new error handling pathway for attempts to remove reminders that have already been completed. It ensures that both text and slash command handlers check for completed reminders and return a localized error message if such an operation is attempted. The changes also include updates to error types, error dictionary management, and localization files for both English and French.

Error handling and command logic:

• Added a check in both the text and slash command handlers to prevent removing reminders that are already completed, returning a specific error if attempted (remove_reminder.rs).
• Introduced a new CommandError::ReminderAlreadyCompleted(String) variant to represent this error case (types.rs).
• Implemented a Display message for the new error to provide a human-readable description (types.rs).

Error dictionary and localization:

• Updated the error dictionary manager to handle the new error type and map it to a localization key with parameters (dictionary.rs).
• Added new localized error messages for the "reminder already completed" case in both English and French message files (en.rs, fr.rs).