Friendly error message when config file not found.

        opts.on('--config=PATH',

          "Loads config options from JSON file.", " ") do |path|

          path = canonical(path)

          if File.exists?(path)

            config = read_json_config(path)

          else

            puts "Oh noes!  The config file #{path} doesn't exist."

            exit(1)

          end

          # treat paths inside JSON config relative to the location of

          # config file.  When done, switch back to current working dir.

          @working_dir = File.dirname(path)