Record command exit status on Execute objects in Phage

Summary:
Ref T2794. With Phacility-specific changes elsewhere, this allows me to add some helpful summary output to the current phage UI.

I'm thinking about having a "plan" (a DOM-like document describing which commands to execute where) produce a "report" document, because a command may actually have multiple exit statuses (for example, if it was automatically retried after a failure). But this makes actually using phage to do things quite a bit easier for now.

Test Plan: Ran some cluster operations and, with changes elsewhere, got more useful high-level reporting about overall command state.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T2794

Differential Revision: https://secure.phabricator.com/D17386