DRILL-2884: 1-Core: Throw "canceled" (not "ResultSet closed") after cancel().
Changed DrillResultSetImpl to throw new ExecutionCanceledSqlException, rather
than (previously existing) AlreadyClosedSqlException, at the first call to the
result set after the associated SQL statement's execution is canceled (except
that currently only some methods check and throw those exceptions).
Also fixed two little documentation errors in DrillResultSet.
