cmd/gb-vendor/vendor: improved failure message
cmd/gb-vendor/vendor: improved failure message

guiquanz/gb,tbruyelle/gb,mopemope/gb,tianon/gb,lparkes/gb,URXtech/gb,ches/gb,tbruyelle/gb,mopemope/gb,acasajus/gb,constabulary/gb,inconshreveable/gb,constabulary/gb,inconshreveable/gb,hyper-carrot/gb,mopemope/gb,constabulary/gb,davidkaya/gb,Luit/gb,davidkaya/gb,MerlinDMC/gb,ches/gb,hyper-carrot/gb,tianon/gb