Show nice error on failure to enumerage input files.
Show nice error on failure to enumerage input files.

Without this we get an ugly stack trace. This is way more
friendly.

Bug: 983006

Change-Id: I5ba871ad71ad43bb48d91697e96473afbfe399a0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1697221
Auto-Submit: Eli Ribble <bf10befcd8d0da915248530ab469b65373a5373c@chromium.org>
Commit-Queue: Robbie Iannucci <40f3d43a28ebae3cb819288542e1c84d73d962d5@chromium.org>
Reviewed-by: Robbie Iannucci <40f3d43a28ebae3cb819288542e1c84d73d962d5@chromium.org>

CoherentLabs/depot_tools,CoherentLabs/depot_tools