pt-fk-error-logger crashes if there's no foreign key error

If there's never been a fk error, then get_fk_error() tries to match /LATEST FOREIGN KEY ERROR/, finds and returns nothing, but then the main loop continues anyway as long as there's an insert sth. It should actually check the return values of get_fk_error(). Easy to reproduce: just spin up a new sandbox and run the tool and you'll get an error about $ts being undefined.