Closes github#822.

In go-sql-driver/mysql#1075, @acharis notes
that the way the go-sql driver is written, query timeout errors don't
get set in `rows.Err()` until _after_ a call to `rows.Next()` is made.

Because this kind of error means there will be no rows in the result
set, the `for rows.Next()` will never enter the for loop, so we must
check the value of `rows.Err()` after the loop, and surface the error up
appropriately.