more informative assert message

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@41662 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/mac/carbon/dccg.cpp

@@ -2033,7 +2033,7 @@ bool wxDC::DoBlit(
     }
     else
     {
-        wxFAIL_MSG( wxT("Blitting is only supported from bitmap contexts") ) ;
+        wxFAIL_MSG( wxT("Blitting is only supported from bitmap contexts, and only with wxCOPY logical operation.") ) ;
         return false ; 
     }