Merge pull request #415 in ZP/z-push from feature/ZP-1093-add-service-unavailable-exception to develop

* commit '749311a1':
  ZP-1095 Use ServiceUnavailableException when Kopano is unavailable.
  ZP-1093 Added ServiceUnavailableException to class loader, updated config description, don't show legal information if service is unavailable, check if config value is greater than zero, throw ServiceUnavailableException if states need to be upgraded.
  ZP-1093 Add service unavailable exception. Released under the Affero GNU General Public License (AGPL) version 3.