stage1: return an error if an app has empty exec
stage1: return an error if an app has empty exec

We were assuming that if "exec" is present, it won't be empty. This is
incorrect because the spec lists exec as optional.

lucasallan/rkt,sinfomicien/rkt,kbrwn/rkt,Wilybald/rkt,jonboulle/rkt,blixtra/rkt,jakon89/rkt,krnowak/rkt,cab105/rkt,s-urbaniak/rkt,rkt/rkt,yifan-gu/rkt,Wilybald/rkt,sgotti/rkt,alban/rkt,krnowak/rkt,sstabellini/rkt,coreos/rkt,sgotti/rkt,jasonchaffee/rkt,lucasallan/rkt,Shashwatsh/rkt,s-urbaniak/rkt,kinvolk/rkt,markllama/rkt,sstabellini/rkt,squeed/rkt,mike1729/rkt,jjlakis/rkt,matthaias/rkt,mike1729/rkt,krnowak/rkt,achanda/rkt,achanda/rkt,akshaykarle/rkt,yifan-gu/rkt,yifan-gu/rocket,intelsdi-x/rkt,blixtra/rkt,monstermunchkin/rkt,markllama/rkt,coreos/rkt,il9ue/rkt,jonboulle/rkt,joshix/rkt,intelsdi-x/rkt,akshaykarle/rkt,il9ue/rkt,linki/rkt,kbrwn/rkt,dgonyeo/rkt,kinvolk/rkt,joshix/rkt,sgotti/rkt,endocode/rkt,endocode/rkt,Wilybald/rkt,rkt/rkt,achanda/rkt,shobhit85/rkt,rkt/rkt,lucasallan/rkt,tmrts/rkt,nhlfr/rkt,nhlfr/rkt,euank/rkt,lucasallan/rkt,dgonyeo/rkt,sinfomicien/rkt,tmrts/rkt,monstermunchkin/rkt,Wilybald/rkt,tmrts/rkt,jjlakis/rkt,endocode/rkt,squeed/rkt,Shashwatsh/rkt,yifan-gu/rocket,yifan-gu/rkt,matthaias/rkt,joshix/rkt,nhlfr/rkt,jjlakis/rkt,s-urbaniak/rkt,il9ue/rkt,ybubnov/rkt,markllama/rkt,jakon89/rkt,rkt/rkt,jasonchaffee/rkt,coreos/rkt,sstabellini/rkt,yifan-gu/rocket,sinfomicien/rkt,Shashwatsh/rkt,mike1729/rkt,euank/rkt,jakon89/rkt,alban/rkt,intelsdi-x/rkt,squeed/rkt,shobhit85/rkt,cab105/rkt,euank/rkt,kinvolk/rkt,dgonyeo/rkt,alban/rkt,blixtra/rkt,linki/rkt,cab105/rkt,shobhit85/rkt,ybubnov/rkt,monstermunchkin/rkt,akshaykarle/rkt,jasonchaffee/rkt,kbrwn/rkt,matthaias/rkt,linki/rkt,kinvolk/rkt,ybubnov/rkt,joshix/rkt,jonboulle/rkt