error out on deleting nonexistent alias
error out on deleting nonexistent alias

All of our other API calls error out when you try to delete something that
doesn't exist, and it seems like this one should too.

Signed-off-by: Tycho Andersen <10c18fcb898ee6b17d6575d444b125d10b3f4d6a@canonical.com>

hallyn/lxd,lxc/lxd,Skarlso/lxd,stgraber/lxd,atxwebs/lxd,ollie314/lxd,dustinkirkland/lxd,lxc/lxd,hallyn/lxd,hallyn/lxd,techtonik/lxd,chrisglass/lxd,techtonik/lxd,lxc/lxd,pcdummy/lxd,chrisglass/lxd,dustinkirkland/lxd,akshaykarle/lxd,erikmack/lxd,stgraber/lxd,pcdummy/lxd,erikmack/lxd,georghopp/lxd,stgraber/lxd,hallyn/lxd,yura-pakhuchiy/lxd,techtonik/lxd,yura-pakhuchiy/lxd,ollie314/lxd,erikmack/lxd,lxc/lxd,jsavikko/lxd,lxc/lxd,jsavikko/lxd,hallyn/lxd,jsavikko/lxd,lxc/lxd,stgraber/lxd,dustinkirkland/lxd,atxwebs/lxd,ollie314/lxd,stgraber/lxd,mjeanson/lxd,chrisglass/lxd,stgraber/lxd,Skarlso/lxd,georghopp/lxd,hallyn/lxd,atxwebs/lxd,lxc/lxd,Skarlso/lxd,mjeanson/lxd,georghopp/lxd,yura-pakhuchiy/lxd,Skarlso/lxd,mjeanson/lxd,akshaykarle/lxd,atxwebs/lxd,pcdummy/lxd,stgraber/lxd,akshaykarle/lxd