config.py: add error code to OpPrereqError

This adds a errors.ECODE_NOENT for OpPrereqError in LookupNodeGroup.

Signed-off-by: Adeodato Simo <dato@google.com>
Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>