support status code hint in panic()
propagate `docker` status code on error
CONFIGURATION_ERROR for missing configuration
IO_ERR for issues when opening config file
DATA_ERR when config unmarshalling fails
USAGE for invalid CLI input detected by cobra
USAGE for unused arguments
USAGE for invalid group/container reference
Merge pull request #30 from bjaglin/statuscodes

Map runtime errors to process status codes so that crane can be used within scripts