If you setup a directory for an org with .chef but don't clone chef-repo, then when you try to vendor a cookbook from the community site you get an error that is hard to understand. Let's make this error easier to understand and tell the user that they should clone chef-repo
[hoop] orgs/my-demo: knife cookbook site vendor tomcat6
INFO: Downloading tomcat6 from the cookbooks site at version 0.4.1
INFO: Cookbook saved: /Users/seth/oc/environments/orgs/my-demo/cookbooks/tomcat6.tar.gz
INFO: Checking out the master branch.
/Library/Ruby/Gems/1.8/gems/chef-0.9.5/lib/chef/mixin/command.rb:181:in `handle_command_failures': git checkout master returned 128, expected 0 (Chef::Exceptions::Exec)
from /Library/Ruby/Gems/1.8/gems/chef-0.9.5/lib/chef/mixin/command.rb:124:in `run_command'
from /Library/Ruby/Gems/1.8/gems/chef-0.9.5/lib/chef/knife/cookbook_site_vendor.rb:58:in `run'
from /Library/Ruby/Gems/1.8/gems/chef-0.9.5/lib/chef/application/knife.rb:115:in `run'
from /usr/bin/knife:19:in `load'