fix(notifications): Notifier::prepare() threw \InvalidArgumentExcepti…

[nickvergessen]

…on which is deprecated

If a user has any notifications this log is generated all the time:

OCA\Deck\Notification\Notifier::prepare() threw \InvalidArgumentException which is deprecated. Throw \OCP\Notification\UnknownNotificationException when the notification is not known to your notifier and otherwise handle all \InvalidArgumentException yourself

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Documentation (manuals or wiki) has been updated or is not required