Check added files have current year in license header

Changed the hook to return a list of errors in the course of the change.
It will be helpful for users to find as many errors in a single run of
linter.

TEST=./pre-upload_unittest.py
TEST=tried to add new file with wrong year and checked linter complains.
BUG=chromium:980061

Change-Id: I2cd0ee95493f47114d62fcb81d72fccc627b8ca1
Reviewed-on: https://chromium-review.googlesource.com/1687034
Tested-by: Keigo Oka <oka@chromium.org>
Commit-Ready: Keigo Oka <oka@chromium.org>
Legacy-Commit-Queue: Commit Bot <commit-bot@chromium.org>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
2 files changed