Aggregations: Added an option to show the upper bound of the error for t...

...he terms aggregation.

This is only applicable when the order is set to _count.  The upper bound of the error in the doc count is calculated by summing the doc count of the last term on each shard which did not return the term.  The implementation calculates the error by summing the doc count for the last term on each shard for which the term IS returned and then subtracts this value from the sum of the doc counts for the last term from ALL shards.

Closes #6696
and maybe it should use the same `readSize` method as `requiredSize`?