Show build results in arc land

Summary:
Fixes T4809. When landing a revision, check for a (non-manual) buildable of the current diff. If we find one, check its status:

• If it passed, print out a message to inform the user that we checked.
• If it failed or is still building, print out details about the issue and require a confirmation to continue.
• Just ignore other cases.

Test Plan:

• Ran arc land on a revision with no buildable, a passing buildable, a failed buildable, and a building buildable for the current diff.
• Got sensible output / prompts.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T4809

Differential Revision: https://secure.phabricator.com/D8801