without error

When using quick access methods for searching rows in UPDATE or 
DELETE there was no check if a fatal error was not already sent 
to the client while evaluating the quick condition.
As a result a false OK (following the error) was sent to the 
client and the error was thus transformed into a warning.

Fixed by checking for errors sent to the client during 
SQL_SELECT::check_quick() and treating them as real errors.

Fixed a wrong test case in group_min_max.test
Fixed a wrong return code in mysql_update() and mysql_delete()