added more descriptive exception when an incorrect image stream is being imported.
added more descriptive exception when an incorrect image stream is being imported.

zmaruo/CocosSharp,haithemaraissia/CocosSharp,netonjm/CocosSharp,MSylvia/CocosSharp,netonjm/CocosSharp,haithemaraissia/CocosSharp,mono/CocosSharp,hig-ag/CocosSharp,MSylvia/CocosSharp,hig-ag/CocosSharp,mono/cocos2d-xna,mono/cocos2d-xna,TukekeSoft/CocosSharp,zmaruo/CocosSharp,TukekeSoft/CocosSharp,mono/CocosSharp