Tiny fix for obuild to print all missing dependencies at once

I'm trying out obuild on a few projects of mine and the current behaviour is a little annoying. That is, giving up on the first not found dependency. I've implemented a tiny fix to make obuild a little more helpful. 

I'm unfamiliar with the codebase/your style so I tried to make the change as unintrusive as possible. So feel free to tell me if something needs to be changed.