When building an AMI image, if grub2 is not available on the system,
the bootloader setup will fail. However, the image builder ignores
this error and proceeds anyway, creating an unbootable image. Fix it
by erroring out if mk-setup-grub.sh fails.
Change-Id: I0c9f69e176513f47be761e0d464484660b9d9856
Reviewed-on: http://photon-jenkins.eng.vmware.com:8082/6339
Tested-by: michellew <michellew@vmware.com>
Reviewed-by: Sharath George