lxc-attach gives a better error message
lxc-attach gives a better error message

Don't assume the user knows the kernel internals :)

Signed-off-by: Daniel Lezcano <3211518dd4d87cb3f7ee12cb52e14b7fb2bc0446@fr.ibm.com>

glensc/lxc,tych0/lxc,rldleblanc/lxc,christiaan/lxc,federicobriata/lxc,abstrakraft/lxc-android-lxc,bostjan/lxc,abstrakraft/lxc-android-lxc,sapun/lxc,aeris/lxc,ojkastl/lxc_Patches,ysbnim/lxc,cjwatson/lxc,lisongmin/lxc,QingweiPeterLan/cs188-lxc,hkjolhede/lxc,ksperis/lxc,raspberrypython/lxc,ioops/lxc,DarknessBeforeDawn/lxc,kylef/lxc,armcc/lxc,inokappa/lxc,rldleblanc/lxc,bostjan/lxc,aeris/lxc,ecarrara/lxc,akshaykarle/lxc,abstrakraft/lxc-android-lxc,ojkastl/lxc_Patches,bmoar/lxc,ss1h2a3tw/lxc,terceiro/lxc,Distrotech/lxc,lazy404/lxc,ss1h2a3tw/lxc,yubo/lxc,weizhenwei/lxc,szaszg/lxc,JohnPeacockMessageSystems/lxc,akshaykarle/lxc,virajs/lxc,sapun/lxc,potatosalad/lxc,federicobriata/lxc,Distrotech/lxc,raspberrypython/lxc,fwilson42/lxc,hfuCN/lxc,szaszg/lxc,kylef/lxc,LynxChaus/lxc,akshaykarle/lxc,ksperis/lxc,regit/lxc,efiop/lxc,tych0/lxc,ysbnim/lxc,Ponce/lxc,usc-isi/lxc,coconutpilot/lxc-gentoo-new,weizhenwei/lxc,thmo/lxc,rowhit/lxc,yubo/lxc,ojkastl/lxc_Patches,terceiro/lxc,jirislaby/lxc,raspberrypython/lxc,ostap/lxc,QingweiPeterLan/cs188-lxc,virajs/lxc,tych0/lxc,ioops/lxc,aeris/lxc,Ponce/lxc,rowhit/lxc,bmoar/lxc,terceiro/lxc,smoser/lxc,szaszg/lxc,hfuCN/lxc,JohnPeacockMessageSystems/lxc,DarknessBeforeDawn/lxc,coconutpilot/lxc-gentoo-new,LynxChaus/lxc,usc-isi/lxc,ojkastl/lxc_Patches,GreatFruitOmsk/lxc,jirislaby/lxc,armcc/lxc,hkjolhede/lxc,major/lxc,bostjan/lxc,christiaan/lxc,cjwatson/lxc,fwilson42/lxc,ostap/lxc,major/lxc,lisongmin/lxc,Pelagicore/lxc,caglar10ur/lxc,Distrotech/lxc,potatosalad/lxc,major/lxc,bmoar/lxc,abstrakraft/lxc-android-lxc,smoser/lxc,cjwatson/lxc,hallyn/lxc,LynxChaus/lxc,ecarrara/lxc,ksperis/lxc,QingweiPeterLan/cs188-lxc,codebauss/lxc,regit/lxc,GreatFruitOmsk/lxc,smoser/lxc,regit/lxc,thmo/lxc,Pelagicore/lxc,Pelagicore/lxc,JohnPeacockMessageSystems/lxc,glensc/lxc,yubo/lxc,ysbnim/lxc,rowhit/lxc,caglar10ur/lxc,GreatFruitOmsk/lxc,ioops/lxc,armcc/lxc,codebauss/lxc,Ponce/lxc,hfuCN/lxc,ecarrara/lxc,usc-isi/lxc,caglar10ur/lxc,lazy404/lxc,rldleblanc/lxc,lisongmin/lxc,jirislaby/lxc,sapun/lxc,federicobriata/lxc,ss1h2a3tw/lxc,thmo/lxc,codebauss/lxc,christiaan/lxc,hkjolhede/lxc,weizhenwei/lxc,hallyn/lxc,efiop/lxc,fwilson42/lxc,virajs/lxc,hallyn/lxc,lazy404/lxc,efiop/lxc,glensc/lxc,smoser/lxc,inokappa/lxc,DarknessBeforeDawn/lxc