Description of changes:

This PR allows Authenticator customers to invoke the errorMap(_:) view modifiers without having to provide an AuthenticatorError for every possible AuthError. Instead, they can just provide a new value only for the errors they want to overwrite and return nil for all the rest, in which case the Authenticator will proceed to handle them with their default behaviour.

For example, if a user only wants to override the message for . notAuthorized when a specific condition is met, they can now do something like:

Authenticator { _ in 
    // ....
}
.errorMap { error in
    if case .notAuthorized = error, anotherCondition {
        return .error(message: "A custom message for .notAuthorized")
    }
    
    return nil
}

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