Show a helpful error when you create a render window and forgot to initialise Root first.
Show a helpful error when you create a render window and forgot to initialise Root first.

--HG--
branch : v1-10

jhu-lcsr-forks/ogre,jhu-lcsr-forks/ogre,jhu-lcsr-forks/ogre,jhu-lcsr-forks/ogre,jhu-lcsr-forks/ogre