get_latest_image.sh: error out when image dirs are missing
get_latest_image.sh: error out when image dirs are missing

Every consumer that I can find of get_latest_image.sh expects this
script to output something useful.  Many don't check the result and
end up spitting out unhelpful messages when it doesn't.  So make a
missing images dir fatal for this script so everyone on top of us
gets this checking for free.

If, in the future, someone actually wants this to output nothing
when there's nothing found, we can add a dedicated flag.  That mode
of operation is a lot less common than having it die by default.

BUG=None
TEST=delete image tree, run ./get_latest_image.sh --board=${BOARD}, see it fail nicely
TEST=delete image tree, run ./image_to_vm.sh --board=${BOARD}, see it fail nicely

Change-Id: I0a2ffd0b9084297b6d2346f02dbdb7bd0ef667bf
Reviewed-on: https://gerrit.chromium.org/gerrit/15589
Reviewed-by: David James <f231dc574bbeb62604660cbc162d1b8642a0a3af@chromium.org>
Reviewed-by: Chris Sosa <959d3d631fb6f17f1b07f751372041a1edb210d1@chromium.org>
Tested-by: Mike Frysinger <8f3f75c74bd5184edcfa6534cab3c13a00a2f794@chromium.org>
Commit-Ready: Mike Frysinger <8f3f75c74bd5184edcfa6534cab3c13a00a2f794@chromium.org>

zhang0137/scripts,nicescale/scripts,endocode/scripts,mjg59/scripts,smilart/scripts,vmware/coreos-scripts,fivethreeo/scripts,glevand/coreos--scripts,vmware/coreos-scripts,vmware/coreos-scripts,trnubo/scripts,smilart/scripts,zhang0137/scripts,crawford/scripts,marineam/coreos-scripts,bcwaldon/coreos-scripts,mischief/scripts,fivethreeo/scripts,gtank/scripts,BugRoger/scripts,sigma/coreos-scripts,endocode/scripts,trnubo/scripts,mischief/scripts,bcwaldon/coreos-scripts,bcwaldon/coreos-scripts,BugRoger/scripts,smilart/scripts,glevand/coreos--scripts,crawford/scripts,mjg59/scripts,mischief/scripts,nicescale/scripts,endocode/scripts,BugRoger/scripts,andrejro/coreos--scripts,andrejro/coreos--scripts,marineam/coreos-scripts,trnubo/scripts,zhang0137/scripts,coreos/scripts,nicescale/scripts,sigma/coreos-scripts,crawford/scripts,sigma/coreos-scripts,andrejro/coreos--scripts,mjg59/scripts,gtank/scripts,coreos/scripts,gtank/scripts,fivethreeo/scripts,marineam/coreos-scripts