Improve usability of Phrequent "Stop Time" dialog

Summary:
Fixes T5848.

• Disallow tracking negative time.
• Preserve note if there's an error with the time selection.
• Show start time and duration.
• Slightly better error messages.

Test Plan: Started and stopped time. Tried to select future/negative ranges.

Reviewers: hach-que, btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T5848

Differential Revision: https://secure.phabricator.com/D10218