Add exception and change getState to isAvailable
Update to modified LogFileStorage interface

* render api/gui errors if isAvailable has error
LogFileStorage error messages are displayed to clients

* retrieval/available states are cached for a period
* updated configuration keys for storage/retrieval retry and delay
* pending_remote state is reported if remote log is not found within a
  grace period after the execution completes
* clean up display in gui for error messages slightly
* handle exceptions via groovy log file storage plugins
Update examples for log file storage interface
Update docs with new configuration and plugin interface info