Have cancel() cause "query canceled" rather than just "ResultSet closed"

After Statement.cancel() is called for a SQL statement's execution, a subsequent call to a method on the ResultSet for that execution yields a "ResultSet already closed" exception. (This isn't fully implemented yet, but it is for, e.g., next().)

A call to cancel() should result in an explicit "execution canceled" exception rather than the usual "already closed" exception (so that it doesn't seem like a Drill programming error rather an expected result of canceling).