If rouge isn't there, say so.

In the previous version, it would say 'redcarpet' wasn't installed,
due to the rescue LoadError block on line 93. This change will tell
the user that, in fact, rouge isn't installed and that this is the
cause of the error, not that redcarpet isn't installed.

Fixes #2464.
https://github.com/jekyll/jekyll/issues/2464

lib/jekyll/converters/markdown/redcarpet_parser.rb

@@ -67,8 +67,15 @@ module Jekyll
                           end
                         when 'rouge'
                           Class.new(Redcarpet::Render::HTML) do
-                            require 'rouge'
-                            require 'rouge/plugins/redcarpet'
+                            begin
+                              require 'rouge'
+                              require 'rouge/plugins/redcarpet'
+                            rescue LoadError => e
+                              Jekyll.logger.error "You are missing the 'rouge' gem. Please run:"
+                              Jekyll.logger.error " $ [sudo] gem install rouge"
+                              Jekyll.logger.error "Or add 'rouge' to your Gemfile."
+                              raise FatalException.new("Missing dependency: rouge")
+                            end
 
                             if Rouge.version < '1.3.0'
                               abort "Please install Rouge 1.3.0 or greater and try running Jekyll again."