post-review fixes as indicated by Serg.

manual testing of error cases done in 5.0 due to support for DBUG_EXECUTE_IF
to insert errors.
Unable to write test case for mysql-test until 5.1 due to support for setting
debug options at runtime.


sql/ha_blackhole.cc:
  update for handler::info() now returning int
sql/ha_blackhole.h:
  update for handler::info() now returning int
sql/ha_isam.cc:
  update for handler::info() now returning int
sql/ha_isam.h:
  update for handler::info() now returning int
sql/ha_isammrg.cc:
  update for handler::info() now returning int
sql/ha_isammrg.h:
  update for handler::info() now returning int
sql/item_sum.cc:
  update for handler::info() now returning int.
  
  return error to user if info call fails
sql/sql_delete.cc:
  update for handler::info() now returning int.
  
  return error to user if info call fails
sql/sql_select.cc:
  update for handler::info() now returning int.
  
  return error to user if info call fails
sql/sql_union.cc:
  update for handler::info() now returning int.
  
  return error to user if info call fails