[r=thumper],[bug=1307290] Fix intermittent destroy error.
[r=thumper],[bug=1307290] Fix intermittent destroy error.

In order to manage the intermittent error that is returned
by os.RemoveAll if the directory is removed by another process
while it is trying, we double check before returning an error.

https://codereview.appspot.com/87500044/
axw/juju,mattyw/juju,dimitern/juju,marcmolla/juju,mattyw/juju,tsakas/juju,mjs/juju,mhilton/juju,tasdomas/juju,mattyw/juju,frankban/juju,kat-co/juju,ericsnowcurrently/juju,alesstimec/juju,mhilton/juju,cloud-green/juju,alesstimec/juju,ericsnowcurrently/juju,Altoros/juju-cloudsigma,ericsnowcurrently/juju,bogdanteleaga/juju,makyo/juju,dimitern/juju,reedobrien/juju,reedobrien/juju,dooferlad/juju,frobware/juju,mhilton/juju,davecheney/juju,cmars/juju,dimitern/juju,waigani/juju,fwereade/juju,bogdanteleaga/juju,cmars/juju,bz2/juju,bz2/juju,perrito666/juju,makyo/juju,marcoceppi/juju,kat-co/juju,gabriel-samfira/juju,AdamIsrael/juju,reedobrien/juju,AdamIsrael/juju,marcoceppi/juju,tsakas/juju,sinzui/juju,thomnico/juju,davecheney/juju,jrwren/juju,voidspace/juju,mikemccracken/juju,gabriel-samfira/juju,tasdomas/juju,mjs/juju,howbazaar/juju,cloud-green/juju,tsakas/juju,tych0/juju,howbazaar/juju,perrito666/juju,frankban/juju,thomnico/juju,gabriel-samfira/juju,bac/juju,mikemccracken/juju,tych0/juju,cherylj/juju,marcmolla/juju,sinzui/juju,rogpeppe/juju,mjs/juju,mhilton/juju-juju,frobware/juju,voidspace/juju,frobware/juju,tasdomas/juju,rogpeppe/juju,ericsnowcurrently/juju,dooferlad/juju,tych0/juju,marcoceppi/juju,mjs/juju,howbazaar/juju,frankban/juju,marcmolla/juju,dooferlad/juju,bac/juju,anastasiamac/juju,mhilton/juju-juju,waigani/juju,AdamIsrael/juju,jrwren/juju,macgreagoir/juju,sinzui/juju,dooferlad/juju,Altoros/juju-vmware,AdamIsrael/juju,gabriel-samfira/juju,axw/juju,macgreagoir/juju,waigani/juju,sinzui/juju,cloud-green/juju,frankban/juju-tmp,Altoros/juju-vmware,alesstimec/juju,frankban/juju-tmp,anastasiamac/juju,mikemccracken/juju,reedobrien/juju,ericsnowcurrently/juju,tasdomas/juju,gabriel-samfira/juju,reedobrien/juju,tych0/juju,bac/juju,waigani/juju,bogdanteleaga/juju,cmars/juju,perrito666/juju,bz2/juju,axw/juju,cloud-green/juju,mattyw/juju,marcmolla/juju,howbazaar/juju,bz2/juju,mjs/juju,Altoros/juju-cloudsigma,wwitzel3/juju,cherylj/juju,kat-co/juju,davecheney/juju,mhilton/juju,mjs/juju,frankban/juju-tmp,voidspace/juju,davecheney/juju,wwitzel3/juju,makyo/juju,thomnico/juju,axw/juju,perrito666/juju,bac/juju,macgreagoir/juju,kat-co/juju,mhilton/juju-juju,marcoceppi/juju,frankban/juju,mikemccracken/juju,anastasiamac/juju,kat-co/juju,mikemccracken/juju,perrito666/juju,dooferlad/juju,bz2/juju,frankban/juju-tmp,fwereade/juju,rogpeppe/juju,Altoros/juju-cloudsigma,Altoros/juju-vmware,Altoros/juju-cloudsigma,marcmolla/juju,anastasiamac/juju,wwitzel3/juju,dimitern/juju,mjs/juju,alesstimec/juju,wwitzel3/juju,Altoros/juju-vmware,alesstimec/juju,axw/juju,dimitern/juju,cmars/juju,bogdanteleaga/juju,makyo/juju,macgreagoir/juju,anastasiamac/juju,voidspace/juju,thomnico/juju,cherylj/juju,tsakas/juju,jrwren/juju,howbazaar/juju,bac/juju,fwereade/juju,cherylj/juju,frankban/juju,mhilton/juju-juju,fwereade/juju,macgreagoir/juju