Add an error message when no package directories are given (#96)
Add an error message when no package directories are given (#96)

Otherwise, if no packages are passed, the script appears to work but
does nothing.
linuxerwang/rules_go,linuxerwang/rules_go,bazelbuild/rules_go,bazelbuild/rules_go,Xjs/rules_go,linuxerwang/rules_go,Xjs/rules_go,Xjs/rules_go,linuxerwang/rules_go,bazelbuild/rules_go,bazelbuild/rules_go,linuxerwang/rules_go,Xjs/rules_go,bazelbuild/rules_go,Xjs/rules_go