This is easy to miss and a toolchain package without win_sdk\Debuggers
will not work as expected so this CL adds a check to stop the
packaging/repacking process as soon as possible.

Bug: 1006238
Change-Id: I272fbaf3e1cbde48a3e260e38e34ffc10d6c86df
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1829272
Commit-Queue: Mirko Bonadei <mbonadei@chromium.org>
Reviewed-by: Bruce Dawson <brucedawson@chromium.org>