ComboGrid: make height for the error configurable

ComboGrid: make height for the error configurable

by introducing a errorHeight config property. This is necessary when
the ComboGrid has e.g. a toolbar and we show the error in the grid body
only, 100 pixels is not enough then. To solve that without hardcoding
different heights, let the subclass/caller configure that

also set this when the store load fails completely (was not done until now)

Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>