devmapper cleanup: improve error msg
devmapper cleanup: improve error msg

1. Make sure it's clear the error is from unmount.

2. Simplify the code a bit to make it more readable.

[v2: use errors.Wrap]
[v3: use errors.Wrapf]
[v4: lowercase the error message]

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

tonistiigi/docker,jameszhan/docker,Mokolea/docker,dims/docker,Ramzec/docker,YuPengZTE/docker,vdemeester/docker,mahak/docker,yosifkit/docker,gnailuy/docker,dmcgowan/docker,paralin/docker,Mokolea/docker,nawawi/docker,Ramzec/docker,unclejack/docker,flavio/docker,jthelin/docker,tophj-ibm/moby,jthelin/docker,jwhonce/docker,jimjag/docker,cpuguy83/docker,SvenDowideit/docker,jimjag/docker,thaJeztah/docker,rancher/docker,x1022as/docker,djs55/docker,YuPengZTE/docker,duglin/docker,trebonian/docker,konstruktoid/docker-1,nalind/docker,tonistiigi/docker,mikebrow/docker,nawawi/docker,yosifkit/docker,mahak/docker,jlhawn/docker,gnailuy/docker,tophj-ibm/moby,ibuildthecloud/docker,laijs/docker,laijs/moby,bearstech/moby,thaJeztah/docker,laijs/moby,tophj-ibm/docker,unclejack/docker,konstruktoid/docker-1,x1022as/docker,jvanz/docker,jlhawn/docker,yosifkit/docker,moby/moby,konstruktoid/docker-1,moul/docker,konstruktoid/docker-upstream,kolyshkin/docker,moby/moby,trebonian/docker,jwhonce/docker,estesp/docker,AkihiroSuda/docker,djs55/docker,gnailuy/docker,laijs/docker,SvenDowideit/docker,cpuguy83/docker,bearstech/moby,dmcgowan/docker,Mokolea/docker,justincormack/docker,tophj-ibm/moby,jameszhan/docker,konstruktoid/docker-upstream,trebonian/docker,moul/docker,bearstech/moby,rremer/moby,laijs/docker,allencloud/docker,moby/moby,unclejack/docker,nawawi/docker,jlhawn/docker,flavio/docker,rancher/docker,AkihiroSuda/docker,nalind/docker,crosbymichael/docker,kojiromike/docker,kolyshkin/moby,noironetworks/docker,kojiromike/docker,djs55/docker,cpuguy83/docker,Mokolea/docker,SvenDowideit/docker,justincormack/docker,jimjag/docker,kolyshkin/moby,moby/moby,paralin/docker,dmcgowan/docker,duglin/docker,bearstech/moby,ibuildthecloud/docker,jvanz/docker,jimjag/docker,crosbymichael/docker,estesp/docker,tmp6154/docker,mikebrow/docker,pradipd/moby,scotttlin/docker,pradipd/moby,jvanz/docker,justincormack/docker,johnstep/moby-moby,yongtang/docker,noironetworks/docker,konstruktoid/docker-upstream,mahak/docker,johnstep/moby-moby,tonistiigi/docker,pradipd/moby,vdemeester/docker,AkihiroSuda/docker,yongtang/docker,kolyshkin/docker,maxamillion/docker,ibuildthecloud/docker,jwhonce/docker,yosifkit/docker,dims/docker,KarthikNayak/docker,nawawi/docker,rremer/moby,yongtang/docker,trebonian/docker,maxamillion/docker,nalind/docker,vdemeester/docker,tophj-ibm/docker,flavio/docker,laijs/moby,allencloud/docker,estesp/docker,cpuguy83/docker,kojiromike/docker,crosbymichael/docker,allencloud/docker,docker/docker,paralin/docker,nalind/docker,ibuildthecloud/docker,Ramzec/docker,tmp6154/docker,scotttlin/docker,kolyshkin/moby,docker/docker,moul/docker,docker/docker,thaJeztah/docker,rremer/moby,KarthikNayak/docker,jameszhan/docker,duglin/docker,mahak/docker,Ramzec/docker,paralin/docker,tophj-ibm/docker,flavio/docker,YuPengZTE/docker,x1022as/docker,moul/docker,rancher/docker,noironetworks/docker,gnailuy/docker,johnstep/moby-moby,konstruktoid/docker-upstream,tmp6154/docker,AkihiroSuda/docker,mikebrow/docker,ibuildthecloud/docker,thaJeztah/docker,KarthikNayak/docker,dims/docker,scotttlin/docker,maxamillion/docker