When no files are found where there should be on the VS packaging script
throw an exception - using a non-existent variable.

Resolving the missing files is a separate problem that I have filed a VS
bug for:
https://developercommunity.visualstudio.com/content/problem/67864/vs-2017-update-3-preview-2-is-missing-mfc-redist-f.html

Bug: 683729
Change-Id: I1a3ba2a342ce7f8fa826300bb808e87c36969b52
Reviewed-on: https://chromium-review.googlesource.com/532114Reviewed-by: Scott Graham <scottmg@chromium.org>
Reviewed-by: Robbie Iannucci <iannucci@chromium.org>
Commit-Queue: Bruce Dawson <brucedawson@chromium.org>