Raise a better error for malformed `git ls-tree`

Summary: Ref T4159. See T4159 for discussion.

Test Plan: Faked the error and generated a reasonable error message.

Reviewers: hach-que, btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T4159

Differential Revision: https://secure.phabricator.com/D7744
Show a queue utilization statistic in the Daemon console

Summary:
This came up recently in a discussion with @lifeihuang, and then tangentally with @hach-que. Make it easier for users to get a sense of whether they might need to add more daemons. Although we've improved the transparency of daemons, it's not easy for non-experts to determine at a glance how close to overflowing the queue is.

This number is approximate, but should be good enough for determining if your queue is more like 25% or 95% full.

If this goes over, say, 80%, it's probably a good idea to think about adding a couple of daemons. If it's under that, you should generally be fine.

Test Plan: {F88331}

Reviewers: btrahan, hach-que, lifeihuang

Reviewed By: btrahan

CC: hach-que, lifeihuang, aran, chad

Differential Revision: https://secure.phabricator.com/D7747