Provide instructions of ruby imports fail in rake

`rake install_prereqs` requires a minimal level of ruby and rake already
installed. If it doesn't exist, then print out a helpful message
indicating next steps.

CHANGELOG.rst

@@ -5,6 +5,8 @@ These are notable changes in edx-platform.  This is a rolling list of changes,
 in roughly chronological order, most recent first.  Add your entries at or near
 the top.  Include a label indicating the component affected.
 
+Common: Make rake provide better error messages if packages are missing.
+
 Common: Repairs development documentation generation by sphinx.
 
 LMS: Problem rescoring.  Added options on the Grades tab of the

rakefile

-require 'json'
-require 'rake/clean'
-require './rakelib/helpers.rb'
+begin
+    require 'json'
+    require 'rake/clean'
+    require './rakelib/helpers.rb'
+rescue LoadError => error
+    puts "Import faild (#{error})"
+    puts "Please run `bundle install` to bootstrap ruby dependencies"
+    exit 1
+end
 
 # Build Constants
 REPO_ROOT = File.dirname(__FILE__)