Wrap everything that can fail in commands into an |action|

This way we'll get unifrom error handling for all command code.
Introduce Command#file_action

This simplifies and de-duplicates code in YbcCommand and RubyCommand
quite a bit.
Use the Messages module for all messages to users
Refactor the Messages module and its usage

The status messages after actions ("OK", "ERROR(ybc)", etc.) are not
printed now. This makes the code more amenable to parallelization (the
lines won't be cut in half if actions run in parallel).
Colorize error and warning messages
Merge pull request #324 from yast/messaging-refactoring-2

[Review] Request from '' @ 'yast/ycp-killer/messaging-refactoring-2'