Add a message to the log entry of an app being disabled

As an admin, it's always a surprise to see that an app got disabled. On
a busy server with many log entries, it's hard to locate the entry that
explains why Nextcloud disabled an app. Adding a message will make it
more obvious, allowing admins and developers to grep for the string.

Signed-off-by: Christoph Wurst <christoph@winzerhof-wurst.at>