WL#2264: Produce a warning if chars were lost when converting to character_set_results

Currently one gets a warning when character set conversion performed on table
insert/update was lossy (i.e. some characters couldn't be represented in target
charset and were replaced with '?')

It would be nice to have a server/connection option to produce a warning if some
characters were lost during conversion to character_set_results (i.e. on select).