Ignore user exception when RejectReadOnly

[thopos]

SQLSTATE '45000' is for user-defined exceptions.
Their errno can collide with system exceptions however.
Don't close the connection on user exceptions.

Description

The connection would wrongly be closed when the config option RejectReadOnly is true and the following stored procedure is called:

DELIMITER $$

CREATE PROCEDURE `CloseConnection`()
BEGIN
    SIGNAL SQLSTATE '45000' 
        SET MYSQL_ERRNO = 1792, MESSAGE_TEXT ='no connection for you';
END$$

DELIMITER ;

Checklist

• Code compiles correctly
• Created tests which fail without the change (if possible)
• All tests passing
• Extended the README / documentation, if necessary
• Added myself / the copyright holder to the AUTHORS file

[thopos]

Abandoned.

Current implementations is in line with similar libraries.