class Berkshelf::NoAPISourcesDefined

def to_s
  "Your Berksfile does not define any API sources! You must define " \
  "at least one source in order to download cookbooks. To add the " \
  "default Berkshelf API server, add the following code to the top of " \
  "your Berksfile:" \
  "\n\n" \
  "    source 'https://supermarket.chef.io'"
end