Improve clang-format error messages when run outside of chromium checkout
Improve clang-format error messages when run outside of chromium checkout

Its useful to be able to run clang-format on projects
outside of chromium. This change informs developers
how they can do that.

Change-Id: I01ef755aa6e5b53a0a5bb6377e92b2ffdfd17eff
Reviewed-on: https://chromium-review.googlesource.com/414204
Reviewed-by: Aaron Gable <bbed39beedae4cdb499af742d2fcd6c63934d93b@chromium.org>
Commit-Queue: Sam Clegg <4504a65e6a180f18042ae97f363f5ae49cc5c461@chromium.org>

CoherentLabs/depot_tools,CoherentLabs/depot_tools