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
I think having both will be confusing. The max error for terms in the top terms gives you an idea of why the returned terms are in the order they are and how out of order they might be (arguably this is better served by the per term error). The max potential count of a missing term helps you understand why a term might have been included in place of another one and how close to dropping off the end the last term was.  I personally think information about the missing term would be more useful as I think generally the absolute ordering of the results is less important than understanding why a result is not in the list at all.