I've got one idea about how to implement this, but it would require adding a new flag field to PyExc_AttributeError type.

This flag, if set, would tell that the AttributeError in question was raised in C descriptor code or under similar circumstances, and that the attribute name was not known, and thus it is OK for setattr/delattr and attribute lookups to append ": attributename" to the end of the message, then clear the flag; then  all those places that raise AttributeError in __get__, __set__, __del__ would just need to set this flag.