Client: Rethrow exception from *_entry with valid messages

Really, throwing "null pointer exception" on entry::io_attribute
is not very good idea as nobody can understand it
Client: Fixed session::set/update/remove_indexes for empty groups list
Merge pull request #296 from shaitan/master

Python: cherry-picked changes from lts.
Merge pull request #297 from euroelessar/master

Fixed exception in entry on error and in set_indexes with empty group list