Show candidate when specified task name is not valid

rake_did_you_mean.rb

begin
  require "did_you_mean"
  module Rake::TaskManager
    def [](task_name, scopes=nil)
      task_name = task_name.to_s
      self.lookup(task_name, scopes) or
        enhance_with_matching_rule(task_name) or
        synthesize_file_task(task_name) or
        (STDERR.puts "Don't know how to build task '#{task_name}' (see --tasks)";
         candidates = DidYouMean::SpellChecker.new(dictionary: @tasks.keys).correct(task_name);
         STDERR.puts "Did you mean?  " + candidates.join(', ') unless candidates.empty?;
         exit 1)
    end
  end
rescue LoadError
end