We could show the rejected hunks  ... then we could handle this in different ways:

* ask for user interaction, e.g. drop user in a shell to fix the changes by hand (with optional update of the /etc/customizepkg.d/NAME patch)
* don't ask for user interaction and fail (which is what we are doing at the moment)

But in general, showing the rejected hunks should be good. I am unsure about deleting them. On the one hand, if we don't delete them the user has to do something about it, and if he doesn't, things could get messy with all thos left over files. On the other hand, if we delete them, its harder for the user to see what actually failed and fix it.