CHROMIUM: make "command not found" QA warnings fatal

Upstream doesn't have a way of making these fatal, only QA warnings.
Make them fatal for all our builds to avoid things slipping through.

BUG=chromium:821145
TEST=CQ passes

Change-Id: I480854b23accfdcee046cb70893e4cff1f2a8b04
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/portage_tool/+/1919635
Reviewed-by: Chris McDonald <cjmcdonald@chromium.org>
Commit-Queue: Mike Frysinger <vapier@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>
2 files changed