Prevent catastrophic data loss when finish_installation.app is not present for any reason

HI Andy - I ran across this when for reasons I won't go into, I ended up with a build of Sparkle.framework that had a missing finish_installation.app. As luck would have it the failure path for this scenario is to continue the effort to copy the finish_installation.app to the App Support folder, but to do so by attempting to write over the host's app support folder itself.