using Log.fatal

patch.rb

if checksum == '7d2742255cf6d5b40ffad091b4f17f390619390e0cf0ad6c64cb5e6655a2f65e'
  #file came from a package install, we gotta replace the file with a patched version
elsif checksum == 'dec8cb0448a046be98b6d7a57b57589fd25556d067a65f4ff9f7d2deb69a7b2e'
  #file already patched, nothing to do
elsif checksum.nil?
  #no file, nothing to do
else
  Chef::Log.fatal("uh oh, what's this checksum? we should probably not replace the file until we know what's going on...")
end

the real recipe actually does stuff ;)