"kpm pack" shows warnings when dependencies are missing

parent #544 

Strictly followed instructions from parent issue #544 . If there are missing dependencies, we output warning and keep working. So it does the same amount of work as previous implementation. `kpm pack` exits with exit code `-1` when there are missing dependencies.
Well, if we keep it the way it is, I think it'll be the first method we've ever shipped with the word `Complain` in its name... :smile: