The scheduler doesn't really need to know in detail which reasons
rendered a node unusable for a node. All it needs from the volume
binder is a list of reasons that it then can present to the user.
This seems a bit cleaner. But the main reason for the change is that
it simplifies the checking of CSI inline volumes and perhaps later
capacity checking. Both will lead to new failure reasons, which then
can be added without changing the interface.
