[OGRE-227] Throw exception when attempting to get an instance batch iterator for a nonexistent material.
[OGRE-227] Throw exception when attempting to get an instance batch iterator for a nonexistent material.

RealityFactory/ogre,RealityFactory/ogre,paroj/ogre,RealityFactory/ogre,paroj/ogre,paroj/ogre,OGRECave/ogre,paroj/ogre,OGRECave/ogre,paroj/ogre,OGRECave/ogre,RealityFactory/ogre,OGRECave/ogre,RealityFactory/ogre,OGRECave/ogre