rkt/image_gc: don't fail hard when unable to get some treestore IDs
rkt/image_gc: don't fail hard when unable to get some treestore IDs

If we can't get some treestoreIDs from the pods, those pods are in a
broken state and we can remove the treestore (if not referenced by some
other healthy pods).

This avoids cases where treestores can't be removed because of damaged
pods.

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