Update issue #183 : If opening file and RenderFile fails, fall back to adding file source and if that fails opening file with async file source.
Forcing render file/add file source/add file source async is still possible with menu items / drag and drop modifier keys
Added error messages if AddSourceFilter or adding file source async fails.

git-svn-id: http://graph-studio-next.googlecode.com/svn/trunk@377 c2f33eb2-41b2-391d-4cb4-d9d0608729ba