aufs: aufsMount: better errors for unix.Mount()
aufs: aufsMount: better errors for unix.Mount()

Signed-off-by: Kir Kolyshkin <3a017b8ddb3f9cf3e4a59978b004111bdeb97f08@gmail.com>

yongtang/docker,flavio/docker,Ramzec/docker,thaJeztah/docker,mikebrow/docker,justincormack/docker,unclejack/docker,moby/moby,jimjag/docker,crosbymichael/docker,jthelin/docker,flavio/docker,djs55/docker,cpuguy83/docker,AkihiroSuda/docker,estesp/docker,vdemeester/docker,rremer/moby,unclejack/docker,Mokolea/docker,mahak/docker,jvanz/docker,bearstech/moby,moby/moby,AkihiroSuda/docker,dims/docker,nalind/docker,rremer/moby,Ramzec/docker,jvanz/docker,dims/docker,nawawi/docker,gnailuy/docker,pradipd/moby,djs55/docker,kolyshkin/moby,Mokolea/docker,cpuguy83/docker,estesp/docker,mahak/docker,moul/docker,pradipd/moby,bearstech/moby,moby/moby,trebonian/docker,pradipd/moby,kojiromike/docker,paralin/docker,kojiromike/docker,konstruktoid/docker-1,paralin/docker,konstruktoid/docker-upstream,paralin/docker,flavio/docker,kolyshkin/moby,flavio/docker,nawawi/docker,trebonian/docker,tmp6154/docker,konstruktoid/docker-1,tmp6154/docker,SvenDowideit/docker,cpuguy83/docker,Ramzec/docker,konstruktoid/docker-upstream,thaJeztah/docker,gnailuy/docker,mahak/docker,konstruktoid/docker-upstream,cpuguy83/docker,mikebrow/docker,ibuildthecloud/docker,kolyshkin/moby,mikebrow/docker,konstruktoid/docker-1,Mokolea/docker,jameszhan/docker,dims/docker,crosbymichael/docker,moul/docker,ibuildthecloud/docker,jimjag/docker,yongtang/docker,yosifkit/docker,nalind/docker,nawawi/docker,Ramzec/docker,Mokolea/docker,jthelin/docker,jimjag/docker,jvanz/docker,unclejack/docker,mahak/docker,bearstech/moby,justincormack/docker,yosifkit/docker,gnailuy/docker,yosifkit/docker,jameszhan/docker,ibuildthecloud/docker,SvenDowideit/docker,yosifkit/docker,docker/docker,thaJeztah/docker,crosbymichael/docker,docker/docker,jameszhan/docker,moby/moby,ibuildthecloud/docker,nawawi/docker,ibuildthecloud/docker,bearstech/moby,thaJeztah/docker,moul/docker,paralin/docker,vdemeester/docker,trebonian/docker,jimjag/docker,kojiromike/docker,justincormack/docker,nalind/docker,konstruktoid/docker-upstream,tmp6154/docker,rremer/moby,yongtang/docker,AkihiroSuda/docker,SvenDowideit/docker,docker/docker,trebonian/docker,djs55/docker,nalind/docker,estesp/docker,gnailuy/docker,vdemeester/docker,AkihiroSuda/docker,moul/docker