Reduce the strength of "arc executing on arc" from an error to a warning

Summary:
See PHI13. This was introduced a very, very long time ago in D311 and D312, and I think T168 was the original report.

It prevents arc from being used in some semi-reasonable (maybe?) automation workflows where you're hooking some version of "Land Revision" up to arc land. This isn't necessarily the right approach, but I think the concession here to make this work is small.

Running arc against another copy of arc makes arc unit not work, but we provide a good error message. Most other arc operations still work correctly.

All of these situations are bizarre edge cases but I think we can safely warn and continue here. Even if we revert this behavior later, almost no one should be affected, since this essentially only impacts users developing arc itself.

Test Plan: Ran one copy of arc against another, saw a warning instead of an error. arc unit failed, but with a good error.

Reviewers: chad, jmeador

Reviewed By: jmeador

Differential Revision: https://secure.phabricator.com/D18264