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The check was never updated to support --gn-paths.

Also adds missing --output arg to error messages when missing.

Bug: 1141255
Change-Id: I1e5fa5b6926247f3f00ae2687c72b894c666e2c0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2491521Reviewed-by: Mohamed Heikal <mheikal@chromium.org>
Commit-Queue: Andrew Grieve <agrieve@chromium.org>
Cr-Commit-Position: refs/heads/master@{#819963}