Bluetooth: Mesh: Skip publish if update fails

Allow models to skip a periodic publish interval by returning an error
from the publish update callback.

Previously, an error return from publish update would cancel periodic
publishing. This can't be recovered from, and as such, no valid model
implementation could return an error from this callback, and there was
no way to skip a periodic publish.

Signed-off-by: Trond Einar Snekvik <Trond.Einar.Snekvik@nordicsemi.no>