[Review] Request from '' @ 'yast/ycp-killer/messaging-refactoring'

Please review the following changes:
  * c583a17 Refactor the Messages module and its usage
  * 2d931b9 Use the Messages module for all messages to users
  * 6881a71 Introduce Command#file_action
  * 63e5237 Wrap everything that can fail in commands into an |action|