[Review] Request from 'aduffeck' @ 'SUSE/machinery/review_140731_improve_error_handling'

Please review the following changes:
  * 96f6b53 Do not continue inspecting the system in case of unexpected errors
  * 136aa85 Handle unexpected errors appropriately
  * 9930c76 Handle "MachineryError"s appropriately
  * af42212 Streamline exception classes and names
  * 6485c64 Remove obsolete exception class