Gracefully skip platforms that aren't yet created.

Unfortunately, the after_prepare hook is being called after each
platform is added, rather than after all are added. We were emitting
warning-ish messages for each copy attempt to the missing platforms, now
we just emit one saying the platform is not yet present.

Eventually, we should also stop repeating customizations for platforms
that have already been done - will be sufficient to leave a file marker
in the platform dir to detect, since platforms are wiped on brand
changes.

Also, clarified the log messages.