Make build_sledgehammer die properly if prereqs are not met.
Make build_sledgehammer die properly if prereqs are not met.

MaximilianMeister/crowbar,rsalevsky/crowbar,tserong/crowbar,rsalevsky/crowbar,mmnelemane/crowbar,rhafer/crowbar,aspiers/crowbar,vwallfahrer/crowbar,aplanas/crowbar,toabctl/crowbar,crowbar/crowbar,dirkmueller/crowbar,aplanas/crowbar,rhafer/crowbar,tserong/crowbar,MaximilianMeister/crowbar,vwallfahrer/crowbar,rhafer/crowbar,nkrinner/crowbar,SUSE-Cloud/crowbar,aspiers/crowbar,crowbar/crowbar,nkrinner/crowbar,SUSE-Cloud/crowbar,toabctl/crowbar,mmnelemane/crowbar,dirkmueller/crowbar