Print friendly error message if the libc tarball is missing.
Print friendly error message if the libc tarball is missing.

Advises the user to try running setup_board.

BUG=chromium-os:18934
TEST=Ran with missing tarball and checked message.

Change-Id: Ice444139188756932734f5f89ef22de4cd12ab41
Reviewed-on: http://gerrit.chromium.org/gerrit/6709
Tested-by: Raymes Khoury <348ad72dc039a127ca70ef38e5de4eebe052b884@chromium.org>
Reviewed-by: Raymes Khoury <348ad72dc039a127ca70ef38e5de4eebe052b884@chromium.org>

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