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