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