Make "arc land" accommodate a minor API change in Harbormaster build statuses

Summary:
See PHI261. Currently "arc land" shows every build staus (passed, failed, building, etc) as yellow. Intended behavior is that passed builds are green, failed builds are red, and so on.

This is because of an unintended API change a while ago in D16356. Since the only impact was a cosmetic color issue, this escaped notice until now.

Additionally, try to use the modern harbormaster.build.search if it is available.

Test Plan:

• Ran arc land with running builds, got reasonable coloration.
• Faked the new method not being available, still got sensible behavior from the old method.

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D18837