sandbox config: Improve validation error message
sandbox config: Improve validation error message

Signed-off-by: Andy Librian <7f788302dda80158a1a740b8ce841cca6fdb08f4@gmail.com>

kubernetes-incubator/cri-o,kubernetes-incubator/cri-o,kubernetes-incubator/ocid,kubernetes-incubator/ocid,nalind/cri-o,nalind/cri-o,kubernetes-incubator/cri-o,nalind/cri-o,kubernetes-incubator/ocid,kubernetes-incubator/cri-o