if there's a Gemfile and doc:guides can't load RedCloth, print instructions for bundler

Author: fxn

railties/guides/rails_guides.rb

@@ -1,6 +1,13 @@
 pwd = File.dirname(__FILE__)
 $:.unshift pwd
 
+# This is a predicate useful for the doc:guides task of applications.
+def bundler?
+  # Note that rake sets the cwd to the one that contains the Rakefile
+  # being executed.
+  File.exists?('Gemfile')
+end
+
 # Loading Action Pack requires rack and erubis.
 require 'rubygems'
 
@@ -20,7 +27,19 @@
   gem 'RedCloth', '>= 4.1.1'
   require 'redcloth'
 rescue Gem::LoadError
-  $stderr.puts %(Generating Guides requires RedCloth 4.1.1+)
+  $stderr.puts('Generating guides requires RedCloth 4.1.1+.')
+  $stderr.puts(<<ERROR) if bundler?
+Please add
+
+  gem 'RedCloth', '>= 4.1.1'
+
+to the Gemfile, run
+
+  bundle install
+
+and try again.
+ERROR
+
   exit 1
 end