Log warning if a file is missing for local coverage

Change-Id: If188b50b9e5e2a945eb90c595192326828a21907
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4559904


Auto-Submit: Prakhar Asthana <pasthana@google.com>
Commit-Queue: Benjamin Joyce (Ben) <bjoyce@chromium.org>
Reviewed-by: Benjamin Joyce (Ben) <bjoyce@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1148279}