Error message is misleading if there are more entries in the clients group than clients

In the following code, the difference of the clients in the group vs the clients themselves is using .abs.

https://github.com/chef/knife-ec-backup/blob/7dd063dedfa3e5101a4f6bc998999780e732bdc0/lib/chef/knife/ec_base.rb#L261

In the case where the clients group has 1 or more clients that no longer exist, then the message show is misleading.

There are 2 missing clients in PROD_ORG's client group file /path/organizations/PROD_ORG/groups/client.json. If this is not intentional do NOT proceed with a restore until corrected. knife tidy backup clean will auto-correct this. https://github.com/chef-customers/knife-tidy

When in fact it should say something like

There are 2 clients in clients in PROD_ORG's client group file /path/organizations/PROD_ORG/groups/client.json that no longer exist.

The way this could happen is if some clients were deleted during the knife-ec-backup, after the client group data was exported but before the client data was exported.

Start at lib/chef/knife/ec_base.rb#L261 and trace the client-group count comparison for a group containing clients that no longer exist. Reproduce the backup case described in the issue and update the error wording so it identifies clients present in the group but absent from the exported client data; the issue is done when this scenario no longer produces the misleading missing-clients message.