--cursor_protocol": fix a misleading error message in case of
SELECT .. INTO.


sql/sql_class.cc:
  Implement select_result::check_simple_select hierarchy to 
  support correct error messages in case of SELECT .. INTO and C API 
  cursors.
sql/sql_class.h:
  Set the error message inside the function that checks for the error
  condition (simple_select, renamed to check_simple_select).
sql/sql_prepare.cc:
  Use a new method that now sets the error.
tests/mysql_client_test.c:
  Add a test case for Bug#24179 "select b into $var" fails with 
  --cursor_protocol" (check for the right error message and error code).